Build: Fix __attribute__((ifunc(...))) detection with clang -Wall.

Now if user-supplied CFLAGS contains -Wall -Wextra -Wpedantic
the two checks that need -Werror will still work.

At CMake side there is add_compile_options(-Wall -Wextra)
but it didn't affect the -Werror tests. So with both Autotools
and CMake only user-supplied CFLAGS could make the checks fail
when they shouldn't.

This is not a full fix as things like -Wunused-macros in
user-supplied CFLAGS will still cause problems with both
GCC and Clang.

CMakeLists.txt

@@ -790,6 +790,14 @@ if(ALLOW_ATTR_IFUNC)
             void func_ifunc(void)
                     __attribute__((__ifunc__(\"resolve_func\")));
             int main(void) { return 0; }
+            /*
+             * 'clang -Wall' incorrectly warns that resolve_func is
+             * unused (-Wunused-function). Correct assembly output is
+             * still produced. This problem exists at least in Clang
+             * versions 4 to 17. The following silences the bogus warning:
+             */
+            void make_clang_quiet(void);
+            void make_clang_quiet(void) { resolve_func()(); }
         "
         HAVE_FUNC_ATTRIBUTE_IFUNC)
     cmake_pop_check_state()

configure.ac

@@ -882,6 +882,14 @@ if test "x$enable_ifunc" = xyes ; then
 		static void (*resolve_func (void)) (void) { return func; }
 		void func_ifunc (void)
 				__attribute__((__ifunc__("resolve_func")));
+		/*
+		 * 'clang -Wall' incorrectly warns that resolve_func is
+		 * unused (-Wunused-function). Correct assembly output is
+		 * still produced. This problem exists at least in Clang
+		 * versions 4 to 17. The following silences the bogus warning:
+		 */
+		void make_clang_quiet(void);
+		void make_clang_quiet(void) { resolve_func()(); }
 	]])], [
 		AC_DEFINE([HAVE_FUNC_ATTRIBUTE_IFUNC], [1],
 			[Define to 1 if __attribute__((__ifunc__()))