Go builds with clang, without cgo module
Sorry, I meant to use ENV.compiler
isl 0.12.1 with more formula cleanups.

* Fix URLs to use new upstream repo and use tag to track stable
  releases.
  - Fix homepage to use freecode site.
  - Fix head URL.
* Now that isl needs to bootstrap before compile.
  - Add :autoconf, :automake, :libtool as build dependencies, and run
    ./autogen.sh before configure.
* Enable verbose build output to help future debugging.
gfortran formula improvement.

* Disable isl and cloog version check to avoid break when they upgrade.
* Enable link-time-optimization (LTO).
python_helper: Comment wording
PythonInstalled: Adding a private_site_packages

that live in the `libexec` dir of a `Cellar`.
PythonInstalled: Allow formulae to set/append PYTHONPATH

Improve robustness of `PYTHONPATH` by first unsetting it (during
`satisfy`) so that the `PythonInstalled` can get the `python.version`
and so forth and then, after that, setting the `PYTHONPATH` to our
`global_site_packages`.
In the `python_helper` we append to the `PYTHONPATH` so if that var has
been set in a formula, it is respected.
Brew audit does no longer complain about setting the
`ENV['PYTHONPATH']`.
doctor: be more helpful with suggestion about PATH

- Show a one liner that will append to
  the user's ~/.bash_profile. In 95% this will be
  ok and we assume zsh people are smart enough
  to know what they have to do.
brew search <user>/<repo> [substring]

For example `brew search homebrew/science` to get a list of all formulae
from that tap, even if not yet tapped.

`brew search <user>/<repo>/<substr>` or
`brew search <user>/<repo> <substr>` to grep for `<substr>`
inside of the tap `<user>/<repo>`.
pil: is now blacklisted.

PIL has been removed and users should use pillow either
by `pip install pillo` or `brew install samueljohn/python/pillow`.