Warn when using Xcode 8 without CLT on 10.11

Use constants for LinkedKegs/PinnedKegs/Locks.

These definitions are scattered throughout the codebase which makes it
hard to refactor them later (my goal is to move them outside of
HOMEBREW_LIBRARY). Unify their definitions for clearer code and easier
movement later.

superenv: help Autotools with 10.12 SDK on 10.11

keg: add ALL_TOP_LEVEL_DIRECTORIES constant.

The GNU Autotools tests for whether a given symbol is defined are
reliably coming to incorrect conclusions on 10.11 with the 10.12 SDK
in Xcode 8. This overrides its decisions by forcing the right answer
in superenv using ac_cv_func_* environment variables and setting them to
"no" on 10.11. The list of problematic symbols is from

  grep 'weak$os10.11' MacOSX.sdk/usr/lib/system/libsystem_c.tbd

utils: link_src_dst_dirs: paths not files.

This is used in diagnostic (and will be used in other places later)
rather than hard-coding other directories later.

This can ask for you to remove directories (e.g.
`/usr/local/share/homebrew`) so don’t call this files to avoid
confusion.

Make scons use stdenv only if immediate build dep

Documentation Updates.

Teach brew to recognize gcc-HEAD as valid compiler

Building gcc with --HEAD results in most of the executables not having a
version suffix, e.g. Building/installing gcc 6 would result in gcc-6,
g++-6, etc. being installed, while building/installing gcc --HEAD would
result in gcc-, g++-, etc. being installed.

The lack of a version suffix prevented brew from recognizing a valid gcc
install, resulting in brew instructing users to install gcc before
building certain formulae even though gcc is installed.

A patch to the gcc formula makes the version number for --HEAD builds
the major version number of the stable version + 1 (7 at this time).
This patch teaches brew to recognize current --HEAD builds as valid
compilers.

cmd/commands: always show developer commands

xcode: 8.0 is no longer prerelease

pkg-config: update zlib for macOS Sierra

Fix cask installation from untapped Tap not working.

formula_installer will use stdenv if scons is anywhere in the formula's
recursive dependency list:

    https://github.com/Homebrew/legacy-homebrew/issues/40401#issuecomment-110066355

Having scons as a dependency should only require stdenv if it is an
immediate build dependency, as otherwise scons shouldn't be invoked.