cmake(pkg-config): resolve CMAKE_INSTALL_PREFIX at install time

not at configure, else this won't work:

    cmake -B builddir ...
    cmake --build builddir ...
    cmake --install builddir --prefix=destdir

as the populated cmake file will not know about the desired prefix

cmake/Modules/GeneratePkgConfig/generate-pkg-config.cmake.in

@@ -57,4 +57,4 @@ set(_interface_compile_options "${_TARGET_INTERFACE_COMPILE_OPTIONS}")
 string(REPLACE ";" " " _interface_compile_options "${_interface_compile_options}")
 
 configure_file("@_pkg_config_file_template_filename@" "@_generate_target_dir@/@[email protected]" @ONLY)
-file(INSTALL "@_generate_target_dir@/@[email protected]" DESTINATION "@CMAKE_INSTALL_FULL_LIBDIR@/pkgconfig")
+file(INSTALL "@_generate_target_dir@/@[email protected]" DESTINATION "${CMAKE_INSTALL_PREFIX}/@CMAKE_INSTALL_LIBDIR@/pkgconfig")