Use _NSGetEnviron on Mac OS X to gain access to environ.

When building nvim as a shared library for testing, environ is not
exposed.  In order to gain access to the environment variables, you must
get a pointer to them from _NSGetEnviron().

It appears that this may affect the FreeBSD platform too.
Only include libintl's include dir if libintl was found.

This prevents an error from CMake when libintl is not found.  It's not a
required library, so we must wrap the inclusion in a conditional.  It
was already done for the library.
Setup better warning flags for Clang.