Support Clang in ax_gcc_func_attribute.m4

As described in commit 813d942, the embedded GCC macro in
`build/ax_gcc_func_attribute.m4` throws false negatives anytime
unrelated warnings are raised, setting `have_func_attribute` to no.

The fix proposed in the macro upstream changed detecting any warnings to
detecting issues related to `-Wattributes` only in order to decide when to
set `HAVE_FUNC_ATTRIBUTE_*`. However, the Clang front-end equivalent is
`-Wunknown-attribute`.

This patch extends ax_gcc_func_attribute.m4 checks to also match the
Clang warning.

Author: athos-ribeiro

build/ax_gcc_func_attribute.m4

@@ -227,7 +227,7 @@ AC_DEFUN([AX_GCC_FUNC_ATTRIBUTE], [
             dnl GCC doesn't exit with an error if an unknown attribute is
             dnl provided but only outputs a warning, so accept the attribute
             dnl only if no warning were issued.
-            [AS_IF([grep -- -Wattributes conftest.err],
+            [AS_IF([grep -- '-W\(unknown-\)\?attributes' conftest.err],
                 [AS_VAR_SET([ac_var], [no])],
                 [AS_VAR_SET([ac_var], [yes])])],
             [AS_VAR_SET([ac_var], [no])])