ftgl: drop an impure -dylib_file on darwin

The build may outright fail because of this on recent macOS:

ftgl> clang-11: error: no such file or directory: '/System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib'

pkgs/development/libraries/ftgl/default.nix

···
     hash = "sha256-6TDNGoMeBLnucmHRgEDIVWcjlJb7N0sTluqBwRMMWn4=";
   };
 
+  # GL_DYLIB is hardcoded to an impure path
+  # /System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib
+  # and breaks build on recent macOS versions
+  postPatch = ''
+    substituteInPlace m4/gl.m4 \
+      --replace ' -dylib_file $GL_DYLIB: $GL_DYLIB' ""
+  '';
+
   nativeBuildInputs = [
     autoreconfHook
     doxygen