sagemathgh-37886: Remove pointless rpaths on macOS; make sage-env polite when run on systems with no toolchain
    
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This PR does the following (after splitting out the Python upgrade to
[sagemath#37914](sagemath#37914)):
1. Fix a comment in the Makefile about how to use the DEBUG flag so it
will actually work.
3. Edit sage-env so that it does not print errors to stderr and/or open
a dialog whenever it is run on a system which has neither XCode nor
command line tools installed.
4. Remove (almost) all uses of rpath on macOS (see below).
5. Fix the primecountpy spkg, which was the only one which actually
needed an rpath in its python extension module.

**About rpaths on macOS**
Sage has been misusing rpaths on macOS for a very long time.  As far as
I can tell, this was an error based on the incorrect assumption that
MACH binaries use rpath in the same way as ELF binaries.

The rpath in an ELF binary provides a search path, beyond the default
search path, where the loader looks for shared libraries needed by the
binary.

A MACH binary specifies the dynamic libraries it needs with load
commands of type LC_LOAD_DYLIB.  The value of a LC_LOAD_DYLIB load
command is a path to a specific library.  There is one such load command
for each library that the binary needs.  The path is an absolute path by
default.  However, the path is allowed to begin with the substring
`@rpath`.  When the binary is loaded that substring is replaced by the
value of each LC_RPATH load command in the binary to produce a list of
absolute paths (or relative paths if the value of the LC_RPATH begins
with `@loader_path`).  Each of the paths on that list is tried as a
possible location of the needed dynamic library.

An obvious consequence of this design is that if none of the
LC_LOAD_DYLIB paths contains the string `@rpath` then there is no need
for any LC_RPATH commands, since they won't be used.  With one exception
(primecountpy) every MACH binary produced when building Sage had
absolute paths for its LC_LOAD_DYLIB load commands.  Nonetheless Sage
was setting the rpath to $SAGE_LOCAL/lib in all of these binaries.  In
fact, it was doing that multiple times, producing many duplicate rpaths
all of which were useless.  (And duplicate rpaths are now deprecated by
Apple's linker.)

This PR removes all of those bogus rpaths.

**About xcode-select**
The sage-env script was calling gcc to find the names of its linker and
archiver in the unlikely case that either of those had a non-standard
name, and it was also calling xcode-select as part of a check to see
whether the selected XCode toolchain had an ld-classic executable.  That
executable was added in XCode 15 as a new name for the old linker when a
new linker was added.  Apple made it possible to use the old linker by
passing the linker option -ld_classic to the gcc command.  The new
linker was buggy in the beginning and was unable to link openblas.  So
numpy and scipy use the old linker.  Sage also wanted to use it.  While
the new linker is probably working well enough now as a linker, it
generates a warning when it encounters duplicate libraries in a link
command.  It also appears that the new linker, or XCode 15 itself,
somehow *creates* duplicate libraries.  While the warnings should be
innocuous, they were causing Sage doctest failures.  So this PR provides
code which adds -Wl,-ld_classic to LDFLAGS in sage-env to replace the
analogous code which was already there.  But the new code is careful to
redirect the error messages to /dev/mull so they do not get printed on
the users terminal whenever sage starts up on a machine which does not
have any XCode toolchain at all.  It also avoids calling gcc in that
case since doing so posts a dialog on the user's screen asking if they
would like to install command line tools.  The dialog is a nice touch,
but should only appear if the user is actually *using* gcc, not as part
of every invocation of Sage, especially since XCode uses the standard
names.

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- [ ] The title is concise and informative.
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- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: sagemath#37886
Reported by: Marc Culler
Reviewer(s): John H. Palmieri, Marc Culler, Matthias Köppe, Volker Braun

Author: Release Manager

build/make/Makefile.in

@@ -17,6 +17,7 @@
 # Always use bash for make rules
 SHELL = @SHELL@
 
+ifndef DEBUG_RULES
 # Check a variable that is only set in build/make/install, but not in sage-env, for example
 ifndef SAGE_PKGCONFIG
 # Set by build/bin/sage-sdist, which invokes the Makefile directly in
@@ -25,6 +26,7 @@ ifndef SAGE_SPKG_COPY_UPSTREAM
 $(error This Makefile needs to be invoked by build/make/install)
 endif
 endif
+endif
 
 # Directory to keep track of which packages are installed - relative to installation prefix
 SPKG_INST_RELDIR = var/lib/sage/installed

build/pkgs/bliss/spkg-install.in

@@ -1,3 +1,7 @@
+if [ "$UNAME" = "Darwin" ]; then
+    LDFLAGS="${LDFLAGS} -Wl,-rpath,${SAGE_LOCAL}/lib"
+    export LDFLAGS
+fi
 cd src
 sdh_cmake -DUSE_GMP=OFF -DCMAKE_VERBOSE_MAKEFILE=ON
 sdh_make

build/pkgs/primecount/spkg-install.in

@@ -1,3 +1,7 @@
+if [ "$UNAME" = "Darwin" ]; then
+    LDFLAGS="${LDFLAGS} -Wl,-rpath,${SAGE_LOCAL}/lib"
+    export LDFLAGS
+fi
 cd src
 
 # Issue #33054: primecount needs "-std=gnu++..."

build/pkgs/primecountpy/spkg-install.in

@@ -1,2 +1,7 @@
+if [ "$UNAME" = "Darwin" ]; then
+    LDFLAGS="${LDFLAGS} -Wl,-rpath,${SAGE_LOCAL}/lib"
+    export LDFLAGS
+fi
+
 cd src
 sdh_pip_install .

build/pkgs/symengine_py/spkg-install.in

@@ -1,2 +1,7 @@
+if [ "$UNAME" = "Darwin" ]; then
+    LDFLAGS="${LDFLAGS} -Wl,-rpath,${SAGE_LOCAL}/lib"
+    export LDFLAGS
+fi
+
 cd src
 sdh_pip_install .

src/bin/sage-env

@@ -370,22 +370,38 @@ if [ -n "$PYTHONHOME" ]; then
 fi
 
 if [ -n "$SAGE_LOCAL" ]; then
-    # On OS X, test whether "ld-classic" is present in the installed
-    # version of the command-line tools. If so, we add "-ld_classic"
-    # to LD_FLAGS. See #36599.
-    # When the full XCode is installed and in use, for example after
-    # "sudo xcode-select -s /Applications/Xcode.app", then "xcode-select -p"
-    # gives "/Applications/Xcode.app/Contents/Developer", but "ld-classic"
-    # is not in the subdirectory "usr/bin/" but rather in the subdirectory
-    # "Toolchains/XcodeDefault.xctoolchain/usr/bin/". See #37237.
-    # However, if LD is set explicitly, as it is within conda on macOS,
-    # do not not do this.
-    if [ "$UNAME" = "Darwin" ] && [ -z "$LD" ] && [ -x "$(xcode-select -p)/usr/bin/ld-classic" -o -x "$(xcode-select -p)/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld-classic" ] ; then
-        LDFLAGS="-L$SAGE_LOCAL/lib -Wl,-ld_classic,-rpath,$SAGE_LOCAL/lib $LDFLAGS"
-    else
-        LDFLAGS="-L$SAGE_LOCAL/lib -Wl,-rpath,$SAGE_LOCAL/lib $LDFLAGS"
+    # Construct and export LDFLAGS
+    if [ "$UNAME" = "Darwin" ]; then
+       LDFLAGS="-L$SAGE_LOCAL/lib $LDFLAGS"
+       # On OS X, use the old linker if it is available.
+       # if "ld-classic" is present in the selected XCode
+       # toolchain, add "-Wl,-ld_classic" to LDFLAGS (see #36599) unless
+       # LD is already set, as it will be with conda on macOS.  When the
+       # selected toolchain is in the Xcode app the output of "xcode-select -p"
+       # is "/Applications/Xcode.app/Contents/Developer", but "ld-classic" is
+       # not in the subdirectory "usr/bin/" but rather in the subdirectory
+       # "Toolchains/XcodeDefault.xctoolchain/usr/bin/".  (See #37237.)
+       if [ -z "$LD" ]; then
+	   # Running xcode-select on a system with no toolchain writes an
+	   # error message to stderr, so redirect stderr to /dev/null. 
+	   XCODE_PATH=$(/usr/bin/xcode-select -p 2> /dev/null)
+	   if [ -n $XCODE_PATH ]; then
+               if [ -x "$XCODE_PATH/usr/bin/ld-classic" -o \
+		       -x "$XCODE_PATH/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld-classic" ]; then
+		   LDFLAGS="$LDFLAGS -Wl,-ld_classic"
+               fi
+           else
+               # On a macOS system with no toolchain we don't want this script
+               # to call gcc because that will also print an error message to
+               # stderr.  We can avoid this by setting AS and LD to their
+               # default values.
+               AS=as
+               LD=ld
+	   fi
+       fi
     fi
     if [ "$UNAME" = "Linux" ]; then
+	LDFLAGS="-L$SAGE_LOCAL/lib -Wl,-rpath,$SAGE_LOCAL/lib $LDFLAGS"
         LDFLAGS="-Wl,-rpath-link,$SAGE_LOCAL/lib $LDFLAGS"
     fi
     export LDFLAGS