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.