Able to build on macOS only with Xcode but without CLT
Created originally on Bitbucket by umireon (Kaito Udagawa)
Was already merged in Bitbucket before import, marked as merged by the import user
In macOS, two different build environment exists: Xcode.app and Xcode Command Line Tools (aka CLT)
Both toolchains ship with libffi and its headers, and thus can build cffi.
By this change setuptools considers the header under Xcode.app, and cffi can build every environment where cc exists.