Reproducible Builds: remove __DATE__ and __TIME__

[ThomasAdam]

For a while now, there's been a patch floating around for FVWM to remove
DATE and TIME macros so that reproducible builds can be achieved.

In trying to track down the original author(s), I've reached out to a few
different people, but no one's been forthcoming or can remember who that was.

So in tribute to them, here's the patch!