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.)

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

Fixes Homebrew/homebrew#1980

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

Treat the "lib/node" folder like Python, Perl, etc.
Create it as a real folder in HOMEBREW_PREFIX, so that
package managers (npm) have a stable place to drop libs.

Changes in this version include:
* Aliases are now defined by relative symlinks rather than in the
  formulae themselves. Many commands are faster now that they don't
  have to read every formula to find aliases.

* "url" now supports the same features as "head", including ":using"
  and version specifiers for VCS systems.

* Files and methods marked for deprecation in 0.7 have been removed.

* The Formula DSL now supports "skip_clean :all" and "keg_only 'reason'"

* Add external command completion
* Add alias completion

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

Specifically, it can accept all the VCS tag specs that head could,
making it more useful for defining stable versions that come from
VCS instead of tarballs.

A new "SoftwareSpecification" class was added to implement this.

This new class holds a "spec" for downloading a software package.
It combines the url (or head url) with the "specs" [1] that head has
been able to take.

This allows both the stable (url) and unstable (head) specification
for a software package to co-exist without stomping on each others
"specs".

[1] "specs" contain instructions on which branch/tag/revision/etc. to use
    from the source repository URL.

optparse adds an "options" method to ARGV, and so does Homebrew.
Rename this method (and remove optparse blocking script) so that
Homebrew plays nicer with external Ruby software.

This fixes the issue where "gem install thin" would break
"brew server", for instance.