curses-compat: Improve probe for request_mouse_pos.

On AIX 7.2, the request_mouse_pos function is defined but it seems
the related macros (e.g., MOUSE_X_POS) don't actually work, leading to
undefined variable references at link time.  The configure test should
have caught this, but it turns out GCC optimizes out the check.

So configure concludes request_mouse_pos is supported and then later
compilation of programs which use this feature fail to link.  Ensure
the macro usage influences the program return value so GCC doesn't
delete it.