Log Message:
Prevent to install pre-modular older Xfixes for native modular X.
* stop to use x11/Xfixes if builtin one is newer (xfixes.pc exists)
* add builtin libXfixes detection.

affect to NetBSD-5, builtin libXfixes is 4.0.3, prevent to install Xfixes 2.0.1,
if it is considerd that builtin Xfixes is not sufficient.