[build] Warn if libgpm is missing and other minor changes.

* On some misconfigured systems, libgpm is required along with ncurses.
  I haven't been able to figure out the correct way to figure out these
  dependencies without resorting to using external tools like 'nm'. So
  for the moment, if libgpm is not found, at least warn the user that
  the build might fail later.

* Ncurses on OS X does not require libgpm, so there is no need to test
  for it.

* There seems to be an 'AC_DEFINE' _BSD_SOURCE which does not seem to be
   used anywhere. Removing this for now.

* Remove some artifacts left behind by Emacs.