Fix the GHC 7.10 build
Fix the GHC 7.10 build

Following e7985ed23ddc68b6a2e4af753578dc1d9e8ab4c9. HasDebugCallStack now
appears in IdInfo, which requires `FlexibleContexts` to be enabled when built
with GHC 7.10.

ezyang/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc