build: use uppercase XRENDER_FOUND

FIND_PACKAGE_HANDLE_STANDARD_ARGS sets <UPPERCASED_NAME>_FOUND
build: changed X11_FEATURE_TYPE to optional on osX

If X11_FEATURE_TYPE is disables on mac xfreerdp can't be build properly.
Setting it to optional allows to enable X11 builds with -DWITH_X11
Merge pull request #1295 from bmiklautz/mac_build_fix

Mac build fix