merge revision(s) 22e4eeda,1ab7c412: [Backport #20515]

	ci: Test whether GMP is working in compilers.yml (#10875)

	Avoid reoccurence of [Bug #20515]

	Requires https://github.com/ruby/ruby/pull/10876 since 18eaf0be

	bug: https://bugs.ruby-lang.org/issues/20515

	RUBY_CHECK_HEADER didn't define HAVE_{header-file} (#10876)

	--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