Problem with building modules requiring Gmp (Gmp, _Image_XFace)

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1689

Reported by Daved Daly, Lightwave Networking daved@lightwave.net

When trying to build the latest Pike 7.2 grabbed from CVS i'm having problems with autoheader/autoconf in the Gmp, and _Image_XFace module directories.

Running autoconf in ./modules/Gmp
configure.in:177: error: undefine: undefined: pike_cv_gmp2___MPN_prefix
acheaders.m4:73: AC_CHECK_HEADER is expanded from...
acheaders.m4:95: AC_CHECK_HEADERS is expanded from...

I'm assuming that's coming from line 62 of ./modules/Gmp/configure.in (or line 57 of ./modules/_Image_XFace/configure.in)

Fwiw. I dont have gmp2 installed on this machine, just gmp-3.1.1.

This being build on an IRIX64 6.5 IP27 machine, with gmp-3.1.1 installed, and using SGI's MIPSPro 7.3.1.2m C and C++ compilers.

This seems similar to bug #150 tho that was for pike 7.0