we might need a better name for ac_nonexistent.h

[Ralf Wildenhues]

Witness: <http://gcc.gnu.org/ml/gcc-help/2008-10/msg00293.html>

I don't know what to suggest, though.
  ac_this_header_should_not_exist_so_an_error_about_it_is_expected.h?

Or maybe a fix is overkill.

Sorry for the noise,
Ralf