Make it easier to control symbol exports

By not hardcoding RDYNAMIC it's possible to control exporting
from the commandline on platforms/compilers that don't support
-rdynamic.

Author: sodero

Makefile

@@ -43,7 +43,10 @@ ifdef FILESDIR
     override CPPFLAGS+=-DFILESDIR=\"$(FILESDIR)\"
 endif
 
-RDYNAMIC=-rdynamic
+ifndef RDYNAMIC
+    RDYNAMIC=-rdynamic
+endif
+
 # Set the CPPCHK_GLIBCXX_DEBUG flag. This flag is not used in release Makefiles.
 # The _GLIBCXX_DEBUG define doesn't work in Cygwin or other Win32 systems.
 ifndef COMSPEC