`-Wl,-z,defs` should be pruned out entirely since `-z defs` is not
understood by macOS's `ld`, and the analogue `-undefined error` is
already the default.
Not quite a mass replacement as I've used OS X and Mac OS X where
describing specific older versions and added compatibility methods
for things in the DSL.
/usr/local/lib is removed from the default linker search path when
SDKROOT is defined or sysroot is specified.
Homebrew sometimes sets SDKROOT without setting a sysroot as of 45e138f.
Move some stuff formerly in `Library/ENV` around:
- Move `Library/ENV/$XCODE_VERSION` to `Library/Homebrew/env/super` as they are
all superenv wrappers and all symlinks to the same version. We never needed
the "separate shims for separate versions" functionality and it just adds
confusion.
- Move `Library/ENV/pkgconfig` to `Library/Homebrew/env/pkgconfig` to get more
things under `Library/Homebrew`
- Move `Library/ENV/scm` to `Library/scm` as these wrappers are not actually
used by or related to superenv (or stdenv) in any way.
