Update README with Apple LLVM/Xcode 5.1 fix

Was having problems getting the project. With @whit537’s help it was narrowed-down to changes in the GCC included with Xcode 5.1 that caused, what used to be a warning, an error that occurs during the build process and causes it to fail.

Updated the README with info on getting through the error, by downgrading the error to a warning. I’m not really down with some Unix lingo so feel free to make wording changes.