libgimpbase: rename constants to gexiv2 constants.
GExiv2 does rename the versions in a different manner, than gimp does.
If a new gexiv2 release is made, the version is altered and after that, the release is made.
So the git master is always the same version as the last release, until the next release is made.
This Gimp branch does compile with the current gexiv2 git master, but not with the current
gexiv2 release (0.10.2).
I expect the next release to be 0.10.3, so configure.ac needs version 0.10.3 of gexiv2.