Provide build time when building from tarball:

When building from tarball (i.g. not from git source tree or when git
is not available) genbuild.sh write undefined $TIME to "build/build.h".

Even worse, when TIME is set in the environment then its value
is written instead of a date.

For me this change fixed FTBFS which I got because I had
TIME enviroment variable set with format for time(1) utility.