Disable optimisation on OSX, as a workaround for QTBUG-5986.
Fix following problem for X11_TYPE= (undefined), NetBSD/amd64 6.1.2.
  dsimple.c:117:14: error: 'XCB_CONN_CLOSED_MEM_INSUFFICIENT' undeclared
Adds following constraint.
+BUILDLINK_API_DEPENDS.libxcb+=  libxcb>=1.9
Thanks wiz@.