glib/gthread-posix: Use cc.compiles() instead of cc.links() for checking for __NR_futex

`cc.compiles()` is minimally faster.

We only want to check here whether `__NR_futex` is defined and don't
want to check anything at link-time.

meson.build

@@ -887,7 +887,7 @@ if host_system == 'qnx'
 endif
 
 # Check for futex(2)
-if cc.links('''#include <linux/futex.h>
+if cc.compiles('''#include <linux/futex.h>
                #include <sys/syscall.h>
                #include <unistd.h>
                int main (int argc, char ** argv) {