CMake: Fix gc_sections genex evaluation in pkgconfig file creation

The gc_sections linker flag was recently wrapped in a
$<CXX_COMPILER_ID> genex to prevent adding it the command line when
using an incompatible compiler.

This causes an issue when generating .pc pkg-config files because
$<CXX_COMPILER_ID> can't be used in the output of a file(GENERATE)
call.

Record the flag in a global property, both the genex-wrapped and bare
forms, so that we can perform a string replacement when generating
the pkg-config file to remove the genex wrapping.

This is not perfect, in the sense that consumers of the .pc file
might get the wrong flag if using an incompatible compiler, but
it's better than outright failing the Qt build.

Distros will be expected to patch the .pc files if necessary.

Note the issue does not usually happen for regular Qt builds because
gc_sections is only enabled automatically for static builds, but for
static builds we don't currently generate .pc files.
So the issue only happens in shared Qt builds where the gc_sections
feature is enabled manually.

Amends a2b6c2f

Pick-to: 6.5 6.6
Fixes: QTBUG-115243
Change-Id: I3f6bdf86c24ee90b6da04994e458b438cc41fc7a
Reviewed-by: Amir Masoud Abdol <[email protected]>

cmake/QtFlagHandlingHelpers.cmake

@@ -139,8 +139,15 @@ function(qt_internal_apply_gc_binaries target visibility)
         elseif(LINUX OR BSD OR WIN32 OR ANDROID)
             set(gc_sections_flag "-Wl,--gc-sections")
         endif()
+
+        # Save the flag value with and without genex wrapping, so we can remove the wrapping
+        # when generating .pc pkgconfig files.
+        set_property(GLOBAL PROPERTY _qt_internal_gc_sections_without_genex "${gc_sections_flag}")
+
         set(gc_sections_flag
             "${clang_or_gcc_begin}${gc_sections_flag}${clang_or_gcc_end}")
+
+        set_property(GLOBAL PROPERTY _qt_internal_gc_sections_with_genex "${gc_sections_flag}")
     endif()
     if(gc_sections_flag)
         target_link_options("${target}" ${visibility} "${gc_sections_flag}")

cmake/QtPkgConfigHelpers.cmake

@@ -51,6 +51,17 @@ function(qt_internal_generate_pkg_config_file module)
     list(TRANSFORM loose_include_dirs REPLACE "${INSTALL_INCLUDEDIR}" "\${includedir}")
     list(TRANSFORM loose_include_dirs REPLACE "${INSTALL_MKSPECSDIR}" "\${mkspecsdir}")
 
+    # Remove genex wrapping around gc_sections flag because we can't evaluate genexes like
+    # $<CXX_COMPILER_ID> in file(GENERATE). And given that .pc files don't support dynamic
+    # evaluation like the $<CXX_COMPILER_ID> genex, distros will be expected to patch the .pc
+    # files according to which compiler they intend to be used with.
+    get_property(gc_sections_with_genex GLOBAL PROPERTY _qt_internal_gc_sections_with_genex)
+    get_property(gc_sections_without_genex GLOBAL PROPERTY _qt_internal_gc_sections_without_genex)
+    if(loose_link_options AND gc_sections_with_genex AND gc_sections_without_genex)
+        string(REPLACE "${gc_sections_with_genex}" "${gc_sections_without_genex}"
+            loose_link_options "${loose_link_options}")
+    endif()
+
     qt_internal_set_pkg_config_cpp_flags(link_options "${loose_link_options}" "")
     qt_internal_set_pkg_config_cpp_flags(compile_defs "${loose_compile_defs}" -D)
     qt_internal_set_pkg_config_cpp_flags(include_dirs "${loose_include_dirs}" -I)