Add the option to link against system-wide libsvm by editing site.cfg

This should make packager's life easier: linking against system-wide libsvm
should be a matter of uncommenting relevant lines in file site.cfg.

Something similar should be done with liblinear.

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@713 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8