__gmp_const removed in recent gmp

[agoode]

Looks like a fix like
http://www.mpfr.org/mpfr-3.1.0/patch08
is needed.

DEBUG: In file included from gc.c:44:0:
DEBUG: gc/int-inf.c: At top level:
DEBUG: gc/int-inf.c:180:43: error: unknown type name '__gmp_const'
DEBUG:                                            __gmp_const __mpz_struct *lhsspace,
DEBUG:                                            ^
DEBUG: gc/int-inf.c:181:43: error: unknown type name '__gmp_const'
DEBUG:                                            __gmp_const __mpz_struct *rhsspace)) {
DEBUG:                                            ^
DEBUG: gc/int-inf.c: At top level:
DEBUG: gc/int-inf.c:257:34: error: unknown type name '__gmp_const'
DEBUG:                                   __gmp_const __mpz_struct *argspace)) {
DEBUG:                                   ^

[MatthewFluet]

Since we require C99 for the rest runtime system, I think that we can simply replace __gmp_const with const in runtime/gc/int-inf.c.