Fix JNI generation on Darwin and XCode 5.1

clang now errors out if an unknown argument is used:
$ cpp -fpreprocessed
clang: error: unknown argument: '-fpreprocessed' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future

This is ugly but works...