diff --git a/var/spack/repos/builtin/packages/gcc/package.py b/var/spack/repos/builtin/packages/gcc/package.py
index b3bb491d7f..2633207113 100644
--- a/var/spack/repos/builtin/packages/gcc/package.py
+++ b/var/spack/repos/builtin/packages/gcc/package.py
@@ -1172,7 +1172,7 @@ def _post_buildcache_install_hook(self):
             os.path.exists(os.path.join(header_dir, h))
             for h in libc.package_class.representative_headers
         ):
-            relocation_args.append(f"-isystem {header_dir}")
+            relocation_args.append(f"-idirafter {header_dir}")
         else:
             tty.warn(
                 f"Cannot relocate {specs_file} include directories, "