build: identify the build time of the binary in the versioning output

Based on a patch by qyot27. Add export LC_ALL=C on top of version.sh to
make the output locale independent.

Note that the build time will not be updated on every "make" invocation,
but only when the git revision is updated. This is a good thing, as
repeated make invocations should not rebuild the binary. (This would
break "sudo make install" too.)