Log Message:
- fine grained X11 dependencies for packages which have either USE_IMAKE
  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.