dpms.h is provided by libXext, but nothing in our configure.ac is
verifying that we have libXext's headers. Fortunately we only need the
definitions in dpmsconst.h (which dpms.h included for us), which is in
xorgproto and thus implied by having an xserver DDK to build against.

And we're even trying to include dpmsconst.h when we think we have it,
meaning when HAVE_XEXTPROTO_71 is defined, but while many other drivers
define that macro in their configure.ac, we for no particularly good
reason do not. Oops. But since xextproto is about ten years old by now
we can probably just safely include it unconditionally.