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

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: 479a57475e ("util: Implement debug-threads for macOS")
Message-Id: <20200716055655.24507-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>