Search for xcbgen with PYTHON_EXECUTABLE
This way users can specify `-D:PYTHON_EXECUTABLE` to force a certain
python executable and that executable will then also be used to search
for xcbgen.
This should also provide a more universal solution to the configuration
issues with pyenv or conda since the user can just specify
`-D:PYTHON_EXECUTABLE=/usr/bin/python3`.
Each major target of polybar can now be enabled/disabled while
configuring (even polybar itself).
The cmake code specific to each target will only run if the target is
enabled.
This allows us to for example just build the documentation without
having to run all the cmake code related to compilation or having the
polybar dependencies installed (other than sphinx).
They were required before, but could technically be deactived (the
compilation would fail, but configuration was possible).
The WITH_XRANDR and WITH_XCOMPOSITE now no longer appear in the cmake
part. It still appears in the source code, but is always set to 1
Fixes#1536
