Closes Homebrew/homebrew#21223.

Fixes Homebrew/homebrew#28144.

Closes Homebrew/homebrew#28173.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

This hack is starting to outlive its usefulness, and produces false
positives.

Fixes Homebrew/homebrew#28147.

This is as Python bindings are not build in a way that can have
system or Homebrew Python used interchangeably.

Allows disabling bottles from requirements.

As pointed out by @mikemcquaid in http://git.io/Q5b9wg



Closes Homebrew/homebrew#28106.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

This reverts commit 557ef8df05a8ea28ed903f08b2009ebc0e218f5a.

Closes Homebrew/homebrew#28095.

Closes Homebrew/homebrew#28094.

MacOS.version#<=> is called many, many times during formula loading with
the same half dozen or so arguments. A typical call to this method
involves:

 * a hash lookup to convert a symbol argument to a string
 * creation of a throw-away Version object wrapping the argument
 * the actual version comparison, which is not cheap

This makes it a prime candidate to be memoized.

The search mechanism in MacOS::Xcode is very slow. It requires shelling
out at least twice, and possibly a third time (in the CLT-only case).

Calling provides_cvs? activates this in order to determine the Xcode
version. But if we know that there isn't an Xcode available for the
current OS that meets the criteria, we can avoid this check entirely.

Now that this is always called with an explicit mode argument, let's
make sure it stays that way.

Closes Homebrew/homebrew#27642.

Sometimes `Dir[]` is used with single files/directories unnecessarily.