We need to have the version string available in multiple places not just
the source code. It is now hardcoded in the root CMakeLists.txt and all
files that need it will be configured with cmake.
This also removed the unecessary duality of GIT_TAG and APP_VERSION and
GIT_TAG_NAMESPACE and APP_VERSION_NAMESPACE.
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.
