Declare dependency on CApiFFI in cabal metadata

This allow to fail early for ghc ports for platforms which
don't support CApiFFI

Follow-up to 219c164

Author: hvr

zlib.cabal

@@ -65,6 +65,8 @@ library
                     DeriveDataTypeable
   if impl(ghc >= 7.2)
     other-extensions: DeriveGeneric
+  if impl(ghc >= 7.6)
+    other-extensions: CApiFFI
   build-depends:   base >= 4 && < 5,
                    bytestring >= 0.9 && < 0.12
   if impl(ghc >= 7.2 && < 7.6)