message: macos: tweak boringssl build config for xcode 9
diff: set -e
  +# TODO(zuercher): Xcode 9 seems to need this or else boringssl doesn't compile
  +if [[ `uname` == "Darwin" ]];
  +then
  + export CPPFLAGS="$CPPFLAGS -D_DARWIN_C_SOURCE"
  +fi
  +
  COMMIT=14308731e5446a73ac2258688a9688b524483cb6 # chromium-61.0.3163.81
  git clone https://boringssl.googlesource.com/boringssl
  