Fix build time when building from tarball (now really)

Don't define BUILD_DATE at all when no git version information
is available. `version.cpp` will then define it for us correctly
to the last commit date.

This has been fixed and broken many times over 0.9 history
(21cc8bd, ef1e984), please don't touch this code unless you plan
on testing all possible scenarios including gitian builds.

Fixes #3570.