cmake sets HAVE_DPMS_PROTOTYPES unconditionally to 1 if we have the dpms
headers. So code inside HAVE_X11_EXTENSIONS_DPMS_H never needs to check for
it.
And since there's only one place that cared for this, we can remove the define
in cmake as well.
Sometimes -lpthread is not enough. This has caused build failures on
Gentoo Linux in the past.
The logic also seems needlessly complex. We check for HAVE_PTHREAD on
UNIX platforms even though CMake errors out if pthreads are not
found. Similarly, we have a fallback for HAVE_PTHREAD_SIGNAL being
false even though we always set it to true.
