gimp: use prefix-relative libdir

Multiple outputs hard-code libdir, which prevents plug-ins to override it in gimpplugindir variable.

pkgs/applications/graphics/gimp/default.nix

@@ -157,6 +157,8 @@ in stdenv.mkDerivation rec {
     "--without-webkit" # old version is required
     "--with-bug-report-url=https://github.com/NixOS/nixpkgs/issues/new"
     "--with-icc-directory=/run/current-system/sw/share/color/icc"
+    # fix libdir in pc files (${exec_prefix} needs to be passed verbatim)
+    "--libdir=\${exec_prefix}/lib"
   ];
 
   # on Darwin,