[RFC PATCH] configure: Fix atomic64 test for --enable-werror on macOS

[Thomas Huth <thuth@redhat.com>]

When using --enable-werror for the macOS builders in the Cirrus-CI,
the atomic64 test is currently failing, and config.log shows a bunch
of error messages like this:

 config-temp/qemu-conf.c:6:7: error: implicit declaration of function
 '__atomic_load_8' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  y = __atomic_load_8(&x, 0);
      ^
 config-temp/qemu-conf.c:6:7: error: this function declaration is not a
 prototype [-Werror,-Wstrict-prototypes]

Suppress the warnings to make it pass.

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 Not sure whether this is the best way to fix this issue ... thus marked
 as RFC.
 Even though the compiler warns here, the program links apparently just
 fine afterwards and CONFIG_ATOMIC64=y gets set in the config-host.mak
 file on macOS, so the 64-bit atomic operations seem to be available...
 Any macOS users here who could shed some light on this?

 configure | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/configure b/configure
index e93836aaae..4d50a07b00 100755
--- a/configure
+++ b/configure
@@ -5939,7 +5939,8 @@ int main(void)
   return 0;
 }
 EOF
-if compile_prog "" "" ; then
+if compile_prog "-Wno-implicit-function-declaration -Wno-strict-prototypes" "";
+then
   atomic64=yes
 fi
 
-- 
2.18.1