Compilation fails with wxWidgets below version 2.9.0

Trying to compile OBS with a wxWidgets version below 2.9.0 causes the following error:

```
window-subclass.cpp: In member function 'virtual wxSize ListCtrlFixed::DoGetBestClientSize() const':
window-subclass.cpp:70:10: error: 'DoGetBestClientSize' is not a member of 'wxControl'
```

According to the wxWidgets API documentation wxWindow::DoGetBestClientSize() was added in version 2.9.0: http://docs.wxwidgets.org/trunk/classwx_window.html#afe8447cb5975584ee3d4a03e3d02ee89

The configure script should probably check for the correct required version and not allow compilation if the required version is not available.