Clean up copyrite.*.

- Make a few functions static, since they are not used outside the file.
- Move pr_difftime() to nrnb.c and make it static there, since that is
  the only place where it was used.
- Move the logic of producing the FFT library identification from
  copyrite.c to fft_*.c, where it naturally belongs.
- master always has a C++ compiler, so remove the unnecessary
  conditional.
- Clean up the include statements in copyrite.c.

Change-Id: I9a72c014f1d0583adacc657414bc8a758d450b21