CMake4.0 Fixes: Only pass sdk to swift when set

CMake 4.0 does not specify CMAKE_OSX_SYSROOT unless it is set
explicitly. When this is the case, passing the `-sdk` flag with an empty
path confuses the compiler and results in build failures.

This patch fixes things to only pass the sdk to the Swift compiler when
the SDK is actually set.

Author: etcwilde

3_bidirectional_cxx_interop/cmake/modules/AddSwift.cmake

@@ -34,11 +34,11 @@ function(_swift_generate_cxx_header target header)
     list(TRANSFORM ARG_SEARCH_PATHS PREPEND "-I")
   endif()
 
-  if(APPLE)
+  if(APPLE AND CMAKE_OSX_SYSROOT)
     set(SDK_FLAGS "-sdk" "${CMAKE_OSX_SYSROOT}")
   elseif(WIN32)
     set(SDK_FLAGS "-sdk" "$ENV{SDKROOT}")
-  elseif(DEFINED ${CMAKE_SYSROOT})
+  elseif(CMAKE_SYSROOT)
     set(SDK_FLAGS "-sdk" "${CMAKE_SYSROOT}")
   endif()
 
@@ -54,7 +54,7 @@ function(_swift_generate_cxx_header target header)
     DEPENDS ${_SwiftSources}
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
     COMMAND
-      ${CMAKE_Swift_COMPILER} -frontend -typecheck
+      ${CMAKE_Swift_COMPILER} -typecheck
       ${ARG_SEARCH_PATHS}
       ${_SwiftSources}
       ${SDK_FLAGS}

3_bidirectional_cxx_interop/cmake/modules/InitializeSwift.cmake

@@ -26,7 +26,7 @@ endfunction()
 function(_setup_swift_paths)
   # If we haven't set the swift library search paths, do that now
   if(NOT SWIFT_LIBRARY_SEARCH_PATHS)
-    if(APPLE)
+    if(CMAKE_OSX_SYSROOT)
       set(SDK_FLAGS "-sdk" "${CMAKE_OSX_SYSROOT}")
     endif()