message: Hide symbols on mac with -fvisibility=hidden
  See Level/leveldown#688
diff: , 'cflags_cc+': [ '-frtti' ]
  }]
  , ["OS == 'mac'", {
  + "cflags+": ["-fvisibility=hidden"],
  'xcode_settings': {
  - 'WARNING_CFLAGS': [
  + "GCC_SYMBOLS_PRIVATE_EXTERN": "YES" # -fvisibility=hidden
  + , 'WARNING_CFLAGS': [
  '-Wno-sign-compare'
  , '-Wno-unused-variable'
  , '-Wno-unused-function'
  