configure: Fix for running with --enable-werror on macOS

The configure script currently refuses to succeed when run on macOS
with --enable-werror:

 ERROR: configure test passed without -Werror but failed with -Werror.

The information in config.log indicates:

 config-temp/qemu-conf.c:3:55: error: control reaches end of non-void
 function [-Werror,-Wreturn-type]
 static void *f(void *p) { pthread_setname_np("QEMU"); }
                                                      ^
And indeed, the return statement is missing here.

Fixes: 479a574 ("util: Implement debug-threads for macOS")
Message-Id: <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>

Author: huth

configure

@@ -4198,7 +4198,7 @@ pthread_setname_np_wo_tid=no
 cat > $TMPC << EOF
 #include <pthread.h>
 
-static void *f(void *p) { pthread_setname_np("QEMU"); }
+static void *f(void *p) { pthread_setname_np("QEMU"); return NULL; }
 int main(void)
 {
     pthread_t thread;