Fix broken build with Xcode 9

[felixrieseberg]

If you're anything like me, Apple just updated your Xcode to version 9, which currently doesn't want to build @paulcbetts/spellchecker. The error isn't wrong, either:

  CXX(target) Release/obj.target/spellchecker/src/spellchecker_mac.o
../src/spellchecker_mac.mm:184:13: error: cannot initialize a variable of type 'NSString *' with an rvalue of type 'const NSString *'
  NSString* globalLang = currentGlobalLanguage ? currentGlobalLanguage : autoLangauge;
            ^            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/spellchecker_mac.mm:185:13: error: cannot initialize a variable of type 'NSString *' with an rvalue of type 'const NSString *'
  NSString* ourLang = this->spellCheckerLanguage ? this->spellCheckerLanguage : autoLangauge;
            ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This PR is tiny and fixes that issue.

[winstliu]

Looks like this is #81 again, but for a different branch?