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.

1 files changed, 8 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9baa314e..c8d52f2d 100644
--- a/CMakeLists.txt
+++ b/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()