Summary:
Cygwin doesn't support -fPIC. Remove it.
Not sure whether we can build shared library in Cygwin but at least it can build without warning.
Test Plan: Build under Cygwin
Reviewers: yhchiang, rven, kradhakrishnan, igor
Reviewed By: igor
Subscribers: leveldb, dhruba
Differential Revision: https://reviews.facebook.net/D40077