maint: use Gnulib libgmp module

maint: use Gnulib libgmp module

This lets use assume multiple-precision arithmetic on all
platforms, simplifying the code.
* bootstrap.conf (gnulib_modules): Add libgmp.
* configure.ac: Don’t call cu_GMP, as this is now done by Gnulib.
* m4/gmp.m4: Remove.
* src/expr.c, src/factor.c: Use gmp.h unconditionally.
* src/factor.c: Use the simpler ‘#ifndef mpz_inits’ to
determine whether there is an mpz_inits macro.