Introduce version.c to limit recompilation on version changes.

Currently version.h includes git-version.h which changes whenever
a different commit is checked out or the working tree becomes dirty
because a file has been modified. Consequently, all .c files that
include this header must be recompiled in this case.

To limit the number of recompilations, this commit introduces
version.c, which contains functions that return the various types of
the version string. It is now the only file that includes version.h,
so only version.o must be rebuilt if git-version.h changes.

This also adds the build date, OS and CC version to the version output.