This reverts commit dc4ca25e0a01e072ba3cf1fc47612aff72c980af.

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

	Avoid reoccurence of [Bug #20515]

	Requires https://github.com/ruby/ruby/pull/10876 since 18eaf0be905e3e251423b42d6f4e56b7cae1bc3b

	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 18eaf0be90. [Bug #20515]

	bug: https://bugs.ruby-lang.org/issues/20515
	follow-up: https://bugs.ruby-lang.org/issues/20494
	follow-up: 18eaf0be905e3e251423b42d6f4e56b7cae1bc3b
	follow-up: https://github.com/ruby/ruby/pull/10805

	test_bignum: defined? returns String (#10880)
	MIME-Version: 1.0
	Content-Type: text/plain; charset=UTF-8
	Content-Transfer-Encoding: 8bit

	didn't verify the test is working properly due to mistaken auto-merge… [Bug #20515]

	bug: https://bugs.ruby-lang.org/issues/20515
	follow-up: 22e4eeda6561693367fc7a00b92b90f46b09cabd
	follow-up: https://github.com/ruby/ruby/pull/10875

Because `optflags` is pasted into the invocation line after `CC`, we were
building with -O1 unintentionally. You can see this in the configuration
summary: https://github.com/ruby/ruby/actions/runs/3933391169/jobs/6727044423#step:9:753

The check actually fails with -O2. To make it pass, upstream suggest
that we use the annocheck GCC plugin. Since it requires building from
source as the debian package for it isn't ready yet, punt on it for now
and use `--skip-gaps`.

Co-authored-by: Jun Aruga <jaruga@redhat.com>

	also disable yjit for clang < 10

	They don't compile.  I guess nobody actively maintain such old
	compilers.  Chances are the situation won't improve.  Let's stop
	testing yjit on them.
	---
	 .github/workflows/compilers.yml | 8 ++++----
	 1 file changed, 4 insertions(+), 4 deletions(-)

	Explain why YJIT is disabled for clang<=9 [ci skip]

	---
	 .github/workflows/compilers.yml | 1 +
	 1 file changed, 1 insertion(+)

	Pass -Werror=lto-type-mismatch for GCC LTO jobs

	This helps to find possible LTO miscompilations earlier. See also
	https://github.com/ruby/ruby/pull/7695.
	---
	 .github/workflows/compilers.yml | 12 ++++++------
	 1 file changed, 6 insertions(+), 6 deletions(-)

	give up checking old compilers

	These old compilers needed old OSes, which are getting EODed.  We
	cannot maintain healthy binary of them (building compilers on our
	own is quite haed).  Let us just retire them.  Fixes [Bug #19353]
	---
	 .github/workflows/compilers.yml | 6 ------
	 1 file changed, 6 deletions(-)

	disable yjit when for non-flonum

	YJIT does not interface with this flag.
	---
	 .github/workflows/compilers.yml | 2 +-
	 1 file changed, 1 insertion(+), 1 deletion(-)

	Note that YJIT requires FLONUM [ci skip]

	---
	 .github/workflows/compilers.yml | 6 +++++-
	 1 file changed, 5 insertions(+), 1 deletion(-)