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?