fix compilation failure on Solaris 11

Authored by kgardas on Mar 28 2016, 5:06 PM.

Description

fix compilation failure on Solaris 11

Summary:
Solaris is quite picky about C and POSIX version combination.
For recent change to C99 we need to switch _XPG6 on which means
_XOPEN_SOURCE should be defined to 600

Reviewers: austin, bgamari, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2053