Summary:
Fixes python3 compatibility issues by replacing filter with a list
comperhension and a potential issue with python2 when override_flags
would be an empty list.

Reviewers: austin, thomie

Reviewed By: austin, thomie

Subscribers: thomie, carter, simonmar, mlen

Differential Revision: https://phabricator.haskell.org/D399

GHC Trac Issues: #9230