grass: fix gdal library issue with r.external command

Upon testing the new commit, found an additional problem with r.external. Adding 'GDAL_DYNAMIC=' to the make and make install steps fixes an error with finding the gdal library for raster layers brought into grass with r.external. See compile instructions that address this here: http://gvsigce.sourceforge.net/wiki/index.php/Compiling_GRASS_GIS