Constructing the path relative to a file in the repository is incorrect
on some supported configurations (i.e., the repository is located in a
different place than the prefix).
ClosesHomebrew/homebrew#24418.
Apparently setting RUBYLIB to '-' causes the library path to be unset. So we need to set our own library path in our scripts.
Fixes Homebrew/homebrew-versions/#47.
1. A minimal build environment, we don't set CFLAGS, CPPFLAGS, LDFLAGS, etc. the rationale being, the less that is set, the less variables we are introducing that can break builds.
2. A set of scripts that replace cc, ld, etc. and inject the -I, -L, etc. flags we need into the args passed to the build-tools.
Because we now have complete control over compiler instantiations we do a variety of clean-up tasks, like removing bad flags, enforcing universal builds and ensuring makefiles don't try to change the order of library and include paths from ones that work to ones that don't.
The previous ENV-system is still available when --env=std is specified.
superenv applies to Xcode >= 4.3 only currently.
