gtk3: Enable separated debug info on Linux

When debugging gtk3 errors or errors in applications that use gtk, it
can be useful to have full debuginfo. This adds debug information in a
separate output so it will not bloat normal systems.

pkgs/development/libraries/gtk/3.x.nix

@@ -75,6 +75,8 @@ stdenv.mkDerivation rec {
     ./patches/3.0-darwin-x11.patch
   ];
 
+  separateDebugInfo = stdenv.isLinux;
+
   mesonFlags = [
     "-Dtests=false"
   ];