Suppress scads of annoying warnings from clang version 14.0.5
(Fedora 14.0.5-1.fc36) of the form "warning: '_Static_assert' with
no message is a C2x extension [-Wc2x-extensions]",
by refusing to use single-arg static_assert with older clang.
* m4/assert_h.m4 (gl_ASSERT_H): Turn -Wc2x-extensions from a
warning into an error when testing static_assert, so that the
annoying diagnostic causes the test to fail, which causes assert.h
to be replaced in a way that suppresses the diagnostic during
the real build.