To support system python on OS X

Some build systems still set the DEVELOPER_DIR to /Developer
and then nothing works any more (xcrun, xcodebuild etc.)
I am looking at you MacVim.

New `depends_on :python` Dependency.
New `depends_on :python3` Dependency.

To avoid having multiple formulae with endings -py2 and -py3,
we will handle support for different pythons (2.x vs. 3.x)
in the same formula.
Further brewed vs. external python will be transparently supported.

The formula also gets a new object `python`, which is false if
no Python is available or the user has disabled it. Otherwise
it is defined and provides several support methods:

python.site_packages # the site-packages in the formula's Cellar
python.global_site_packages
python.binary # the full path to the python binary
python.prefix
python.version
python.version.major
python.version.minor
python.xy # => e.g. "python2.7"
python.incdir # includes of python
python.libdir # the python dylib library
python.pkg_config_path # used internally by brew
python.from_osx?
python.framework?
python.universal?
python.pypy?
python.standard_caveats # Text to set PYTHONPATH for python.from_osx?
python.if3then3 # => "" for 2.x and to "3" for 3.x.

Further, to avoid code duplication, `python` takes an optional
block that is run twice if the formula defines depends_on
:python AND :python3.

python do
  system python, 'setup.py', "--prefix=#{prefix}"
end

Read more in the Homebrew wiki.

Fixes:

$ HOMEBREW_CC=gcc brew irb
...
irb(main):001:0> require 'superenv'
=> true
irb(main):002:0> ENV.setup_build_environment
=> nil
irb(main):003:0> ENV.setup_build_environment
Warning: Invalid value for HOMEBREW_CC: gcc-4.2

open-uri on Ruby pre-1.8.6 simply will not open https URLs at all,
making it impossible to check the Github API using the same method
as Homebrew.

This may only be disabled temporarily until the issue search is
rewritten, for example to use the curl helper.

If we're using a homebrewed gcc-4.2, xcrun may fail to find it (or,
worse, find superenv's shim instead). Explicitly add it to the PATH and
search all path elements for the requested tool.

Also make sure to specify 'gcc-4.2' as the compiler name, not plain
'gcc'. That can resolve to llvm-gcc and to gcc-4.0 on various Xcodes.

Closes Homebrew/homebrew#17935.

This fixes superenv on Xcode 3.2.6, and also fixes C++ software with
gcc-4.2 on all Xcodes.

Fixes Homebrew/homebrew#20080.

Due to the precedence of "and" relative to "||", this was not working as
intended; but because #version influences #prefix, the outcome was still
correct. So we can simplify this method quite a bit, and take the
opportunity to share code with #prefix.

Fixes Homebrew/homebrew#20084.

Closes Homebrew/homebrew#20081.

This matches the eql? definition for requirements.