[GRASS5] datum shifts again

[Markus Neteler]

Hi,

would it be difficult to extend
 src/libes/proj/datum_shift.c
to use any datum from the PROJ_INFO file? I am continuously
getting nice results with 'cs2cs' from PROJ4 - so the
datum transform software seems to be in a good condition.

BTW: I have fixed a hardcoded /usr/local/grass5 in datum_shift.c.
This should go to the release branch.

Markus