FindPkgConfig: Update cache variables after isystem extraction

Fixes: #25377

Modules/FindPkgConfig.cmake

@@ -667,6 +667,9 @@ macro(_pkg_check_modules_internal _is_required _is_silent _no_cmake_path _no_cma
 
       if (${_prefix}_CFLAGS_OTHER MATCHES "-isystem")
         _pkgconfig_extract_isystem("${_prefix}")
+        # Using _pkgconfig_set in this scope so that a future policy can switch to normal variables
+        _pkgconfig_set("${_pkg_check_prefix}_CFLAGS_OTHER" "${${_pkg_check_prefix}_CFLAGS_OTHER}")
+        _pkgconfig_set("${_pkg_check_prefix}_INCLUDE_DIRS" "${${_pkg_check_prefix}_INCLUDE_DIRS}")
       endif ()
 
       _pkg_recalculate("${_prefix}" ${_no_cmake_path} ${_no_cmake_environment_path} ${_imp_target} ${_imp_target_global})