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
This patch enables support for nl80211. In case the libnl-genl-3.0
library isn't found, it will fall back to Wext instead.
The library to use can also be manually set with the CMake option
WITH_LIBNL.
The Wireless-Extensions (WE or Wext) are deprecated and long replaced
by cfg80211.
Although Wext isn't used by WiFi drivers anymore, CFG80211_WEXT allows
old tools to communicate with modern drivers by providing a wrapper
API.
