fix compilation failure on Solaris 11
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

vTurbine/ghc,sdiehl/ghc,ezyang/ghc,vikraman/ghc,ezyang/ghc,ezyang/ghc,olsner/ghc,snoyberg/ghc,vikraman/ghc,sdiehl/ghc,tjakway/ghcjvm,mettekou/ghc,ezyang/ghc,olsner/ghc,olsner/ghc,vTurbine/ghc,tjakway/ghcjvm,tjakway/ghcjvm,vTurbine/ghc,sdiehl/ghc,shlevy/ghc,mettekou/ghc,mettekou/ghc,sgillespie/ghc,snoyberg/ghc,vikraman/ghc,vikraman/ghc,vikraman/ghc,sdiehl/ghc,ezyang/ghc,mettekou/ghc,sgillespie/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,vikraman/ghc,tjakway/ghcjvm,vTurbine/ghc,snoyberg/ghc,mettekou/ghc,sdiehl/ghc,sgillespie/ghc,sdiehl/ghc,tjakway/ghcjvm,vTurbine/ghc,snoyberg/ghc,olsner/ghc,sgillespie/ghc,shlevy/ghc,snoyberg/ghc,sgillespie/ghc,vTurbine/ghc,vikraman/ghc,olsner/ghc,ezyang/ghc,tjakway/ghcjvm,olsner/ghc,shlevy/ghc,olsner/ghc,tjakway/ghcjvm,vTurbine/ghc,mettekou/ghc,snoyberg/ghc,snoyberg/ghc,sgillespie/ghc,mettekou/ghc,sgillespie/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc