This should be a temporary workaround until we get a grip on some
Mavericks/Xcode 5.0 issues with e.g. ghc and go (stuff in core
that currently needs GCC at runtime). These formulae both have
Clang fixes in development.

Things that needs GCC at compile-time should be booted from core.

References Homebrew/homebrew#22872.
References Homebrew/homebrew-dupes#229

Any formula with a URL that ends in ".xz" now gets this dependency
automatically, so this check no longer does anything.

Otherwise it's possible that certain buildtime dependencies, that won't
be linked against, will block a build.

See Homebrew/homebrew#23115.

Copy these files/folders to a .bottle directory under the Cellar for installation
elsewhere at pouring time.

Temporarily disable HOMEBREW_GIT_ETC methods until fixed.

This method allows copying a file to a new location by performing a
substitution on the pathname.

If an etc file exists on installation instead of overwriting it (or
requiring all the manual checks in formula) simply copy it with the
extension `.default` appended.