CMake: Make generated pkgconfig files relocatable

Don't write an absolute prefix into generated .pc files but one that's
relative to ${pcfiledir}. This makes the files relocatable, and we don't
have to patch them in the Qt online installer.

Pick-to: 6.8 6.9
Fixes: QTBUG-136210
Change-Id: I5c2f9398917be03f6c63286e553c87ff52971285
Reviewed-by: Alexey Edelev <[email protected]>

Author: jobor

cmake/PkgConfigLibrary.pc.in

@@ -1,4 +1,4 @@
-prefix=@CMAKE_INSTALL_PREFIX@
+prefix=${pcfiledir}/@relative_install_prefix@
 exec_prefix=${prefix}
 bindir=${prefix}/@INSTALL_BINDIR@
 libexecdir=${prefix}/@INSTALL_LIBEXECDIR@

cmake/QtPkgConfigHelpers.cmake

@@ -125,6 +125,9 @@ function(qt_internal_generate_pkg_config_file module)
     qt_path_join(pc_step1_path "${build_dir}" ${step_prefix}_step1.pc)
     qt_path_join(pc_step2_path "${build_dir}" ${step_prefix}_$<CONFIG>_step2.pc)
 
+    file(RELATIVE_PATH relative_install_prefix "/${path_suffix}" "/foo")
+    get_filename_component(relative_install_prefix "${relative_install_prefix}" DIRECTORY)
+
     configure_file("${template_pc}" "${pc_step1_path}" @ONLY)
 
     file(GENERATE OUTPUT "${pc_step2_path}" INPUT "${pc_step1_path}")