Update Autoconf tests for recent GMP version

This should improve the check for a suitable installed GMP version, to
implicitly check if GMP provides __gmpz_powm_sec() which has been added
in GMP version 5.0.0 and switch to the in-tree GMP 5.0.3 source otherwise.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>