Add GAMBIT_GSC_BACKEND and C directive to Gambit docs
Add GAMBIT_GSC_BACKEND and C directive to Gambit docs

mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm,mmhelloworld/idris-jvm