Remove obsolete/redundant FLEXIBLE_ARRAY macro

Authored by hvr on Apr 18 2016, 3:26 PM.

Description

Remove obsolete/redundant FLEXIBLE_ARRAY macro

This macro is doubly redundant, first off all, ancient GCCs prior to
version 3.0 are not supported anymore, but more importantly, we require
a ISO C99 compliant compiler, so we can use the proper ISO C syntax
without worrying about compatibility.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D2121