grass startup script: Add a new config option `libgis_revision`

VERSIONNUMBER which actually refers to ...? Whole include directory, i.e., the whole C/C++ API/ABI I guess. The other candidates I can think of are gis.h, libgrass_gis.so source code, or GRASS version.

We can add the new name now. We just have to keep the old one as @landam suggests.

_Originally posted by @wenzeslaus in https://github.com/OSGeo/grass/pull/532_