This will print installed brews that are missing deps.

This command generates a GraphViz dot file from the Hoembrew
dependency graph.

$ brew install graphviz
$ brew graph | dot -Tsvg -ohomebrew.svg
$ open homebrew.svg

Passing --installed will limit the list to formulae that you have installed.
Documented this and "brew deps --1" which limits the dependency list to
one level down (direct dependencies).

Add a check to see if any keg_only brews are installed and linked
into the Cellar. Doing this can cause installs of other formulae
to fail.

If you symlink /usr/bin/cc to llvm, then Homebrew's "fails_with_llvm"
checks are bypassed. So add a warning to "brew doctor".

Tweak wording in X11 check and use the same X11 detection function
as ENV.x11; also move that function into utils.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Fixes Homebrew/homebrew#2089

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

(This is needed for 10.5 support.)