assert-h: static_assert is a keyword in C23

* m4/assert_h.m4 (gl_ASSERT_H): Also test for static_assert
keyword a la C23, and define HAVE_C_STATIC_ASSERT if so.
If not, arrange for config.h to #define static_assert
by including <assert.h>, and then do "#undef assert"
so that the assert macro still needs an explicit include.
This should be safe even on very old hosts, as assert.h
has been re-includable for decades.
* tests/tests-assert.c: New test.
* modules/assert-h-tests (Files, Makefile.am): Add it.

modules/assert-h

@@ -1,5 +1,5 @@
 Description:
-An <assert.h> that conforms to C11.
+An <assert.h> and static_assert that are like C23.
 
 Files:
 lib/assert.in.h