Or if you don't want to change a global environment variable, you can start aMule like this:
$ LD_LIBRARY_PATH=/usr/wxWidgets/lib/ amule &
This is what I do, because I have more than one version of wxWidgets installed in my system. Depending on the version of aMule/wxWidgets I am testing, I just change the directory in the LD_LIBRARY_PATH path.