MAINT: runtests: make it work on Fedora

`runtests.py` looks into a wrong directory on Fedora linux (and derivatives), which patch Python to install modules into a `lib64` rather than `lib` directory. The flag has no effect on vanilla Python.

Fixes an issue reported on the mailing list: 
http://permalink.gmane.org/gmane.comp.python.numeric.general/54947