Fiddle LD_LIBRARY_PATH to include GDAL

Some GRASS functions dlopen GDAL, and don't get patched in the install.
To make them work I have to export LD_LIBRARY_PATH from within grass again with pkgs.gdal/lib.
Alternatively this does the trick - there might be a better way to do this, if the configure script can take the true path to gdal and reflect it in the dlopen calls or something?

pkgs/applications/gis/grass/default.nix

@@ -60,6 +60,7 @@ stdenv.mkDerivation {
     wrapProgram $out/bin/grass70 \
     --set PYTHONPATH $PYTHONPATH \
     --set GRASS_PYTHON ${python2Packages.python}/bin/${python2Packages.python.executable}
+    --suffix LD_LIBRARY_PATH ':' '${gdal}/lib'
     ln -s $out/grass-*/lib $out/lib
   '';