--with-gmp is not working at all because HAVE_GMP_H
was missing since 18eaf0be. [Bug #20515]

bug: https://bugs.ruby-lang.org/issues/20515
follow-up: https://bugs.ruby-lang.org/issues/20494
follow-up: 18eaf0be
follow-up: https://github.com/ruby/ruby/pull/10805