[StGit] Issue when using custom --prefix on 64bit-system

Hi there,

unless I'm doing something wrong (using "make prefix=$HOME/... install"), I think stg has a small glitch on some lib64-systems.

For me it complains that it can't find stgit.main, which is no surprise, since the sys.path only contains the 'lib' subdirs added in lines 34-36, while for me the stgit-libs where installed in $PREFIX/lib64 (this is on Gentoo AMD64 stable).

The simple fix would be to just extend the local_path with the same entries with "lib64", but I'm sure there is a better way ;)

Greetings, Jojo