libXext/buildlink3.mk, now that it is included there.
Leave the places where its API version is set or variables from it
are used directly (about 3 packages).
or USE_X11BASE set, but don't include mk/x11.buildlink3.mk directly or
via buildlink3.mks
- introduce BUILDLINK_PREFIX.libXpm as alias for BUILDLINK_PREFIX.xpm
in the !modular case
- fix some cases where the check for libX11 couldn't work at all by using
C++ for compilation without including the proper headers
Verified using a full X11_TYPE=xorg bulk build without additional
breakage. Discussed with salo@, wiz@ and send to packages@ for feedback.
