[CMake] Last minute build-date file.

[william.crocker at analog.com]

> 
> But if you're going to write a C program anyway....
> 
> ...then just use "time" and/or "gmtime" standard library functions to 
> print the date/time out in whatever format you want.
> 

The hard part here is not printing out the date and time.
The hard parts is capturing the date and time AT link time,
freezing those numbers and incorporating them into the executable.
All in a portable way and without forcing a relink.

I think I have enough pieces from the various responses that
I can now assemble the final solution.

Bill