graphicsmagick: set full paths to delegates such as ghostscript

The --with-frozenpaths is something that we already set
unconditionally for imagemagick and imagemagick7. Without it
graphicsmagick may not be able to find its runtime dependencies.

pkgs/applications/graphics/graphicsmagick/default.nix

@@ -17,6 +17,7 @@ stdenv.mkDerivation rec {
 
   configureFlags = [
     "--enable-shared"
+    "--with-frozenpaths"
     "--with-quantum-depth=${toString quantumdepth}"
     "--with-gslib=yes"
   ];