Fix generation of version_git.h for some versions of BSD make.

Fix generation of version_git.h for some versions of BSD make.

In order to support VPATH builds, we have to use ${srcdir}/version.c as
the target for the rule that depends on the generation of version_git.h.
When not doing a VPATH build, ${srcdir} expands to ".", so the target
will be "./version.c". However, on some BSDs, make does not understand
that "./version.c" is the same as "version.c", and therefore it doesn't
trigger generating version_git.h when trying to build version.o. (It
works fine if you do a VPATH build, and OpenBSD's make does the right
thing in all cases.)

The trick is to have version.c depend on ${srcdir}/version.c. Of course,
Linux's make knows this is nonsense and will complain about a circular
dependency, so add this rule only on BSD platforms.