Fixed build issue that was caused by not having both the 2.9 and 3.0 versions of wxWidgets installed

`wx-config` returns an error that wasn't being caught when passed a `--version=*` flag specifying a version of wxWidgets that has not been installed.

This patch wraps the call to `wx-config` and returns an empty string on failure so that it will be appropriately discarded by the `isPrefixOf` function.

This was tested on Ubuntu 14.04 with only the `libwxgtk3.0-dev` and `libwxgtk-media3.0-dev` wxWidgets packages installed.