Fix access to environ on OS X

According to the environ manpage on OS X:

  Shared libraries and bundles don't have direct access to environ,
  which is only available to the loader ld(1) when a complete program
  is being linked.

Access to environ can be obtained via the _NSGetEnviron() function. Without defining this, the build fails with a missing symbol error (see https://github.com/mxcl/homebrew/pull/20278).