GitHub #219, #221: Updates to wscript for python3 and LD_LIBRARY_PATH

This commit re-applies be5ede4 but only conditionally calls
decode() when the string is not a str (in py3, it will be an
instance of bytes)

The other bit (#221) is to only conditionally modify
LD_LIBRARY_PATH in the activate script.  This should prevent
the path getting polluted.