Include architecture-neutral metadata in the x64 sysroot.

.gitea/scripts/install-linux-build-dependencies.sh

@@ -79,6 +79,13 @@ case "$target" in
         for package in "$archives"/*.deb; do
             sudo dpkg-deb --extract "$package" "$sysroot"
         done
+
+        # Architecture-independent development packages may already be
+        # installed on the host, so apt does not download them again. Their
+        # pkg-config files contain no host binaries and are safe to reuse with
+        # PKG_CONFIG_SYSROOT_DIR, which redirects every path into this sysroot.
+        sudo mkdir -p "$sysroot/usr/share/pkgconfig"
+        sudo cp -a /usr/share/pkgconfig/. "$sysroot/usr/share/pkgconfig/"
         ;;
     *)
         echo "unsupported Linux build target: $target" >&2