Update grass.rb

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

Closes #24320.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>