Fix OS X build failure due to a deprecated method.

Fix OS X build failure due to a deprecated method.

Apple upgraded me to Xcode 7 yesterday, and now [NSString cString]
gives a deprecation warning, which -Werror turns into a full-on build
failure. Explicitly specify an encoding.

(I mention in a comment that there's an alternative piece of API that
I possibly ought to be using instead, but until I make a concrete
decision about where my backwards compatibility threshold is, I'll
leave it as it is for the moment.)