sed error on FreeBSD due to GNU extensions

[yggdr]

virtualenvwrapper uses GNU extension like \s in sed regexes instead of POSIX [[:space:]]. This makes it fail on non-GNU systems like FreeBSD.

Possible solutions:

1. replace GNU extensions with POSIX ones
2. check for the presence of GNU sed by the name of gsed (which is available in packages on Free- and OpenBSD) and use that if available