ENV.x11: always add include/freetype2 to CPPFLAGS
ENV.x11: only use SDK when CLT is missing and X11 is from Apple
superenv: fix hardcoded X11 aclocal path
Check types in Formula#==

Comparing two objects should not raise an exception, even if they have
different types.

The semantics of #== are now the same as #eql?, so make one an alias.