Make libxbee support optional.

Libxbee support can now be disabled by doing `qmake DEFINES+=DISABLE_XBEE`.
Linux requires libxbee to be installed to system directories. This is now auto-detected, so if the developer has not installed libxbee it automatically skips using it.
Details on why libxbee support is not compiled is now provided in output messages.

Note that with these changes libxbee is expected to be installed on Linux. I don't know if this was required before, but it's the "right" way to do it and the old way of trying to link directly didn't allow me to run the resulting executable because it couldn't find libxbee. Installing libxbee fixed this.