install headers into wx release directory

* where wx release directory is for example, wx-2.8
* WX_VERSION passed in at cmake time with -DWX_VERSION
* also move custom setup.h from wx repo to wxcmake repo
* attempting to support multiple versions of wx at same time (2.8 and 2.9)
directory tweaks, supporting multiple wx versions
Merge remote-tracking branch 'origin/wx28' into wx29