Simplify `Hbc::SystemCommand::Result#plist`.

link: stop unneeded force linking on Mojave/CLT 10.

People are getting in the habit of force-linking things like `zlib` to
fix linking/include issues on Mojave (which doesn't install headers to
`/usr/include` by default). This way lies madness so encourage people to
instead pass the correct compiler flags instead.

Update manpage with information about alternative compilers

CLT: correct header package bundle ID

Remove unused `incremental_hash` method.

* change `MacOS` -> `macOS`
* regenerate manpages

linkage: fix output of optional dependencies.

Generate new man pages so that the docs and manpages match up

This fixes the rubocop offences, which was blocking Travis

* update references in `--cc` flag, using modern gcc examples
* note that issues should not be filed if `--cc` flag is used
* remove reference to `HOMEBREW_CC` as it should not be exposed to users
* generate manpages using `brew man` with these changes

formula_installer: correctly handle build dep reqs.

download_strategy: use tar xf.

The flags and separate `-` aren't required on 10.5 which is the oldest
version of macOS we support (and it looks nicer this way).

When a build dependency has a requirement it should be ignored when
checking for valid requirements. This requires calculating the
dependencies for a formula again and checking them when calculating
which requirements should be pruned.

extend/ENV: support CX11 for LLVM Clang.

Check the runtime dependencies from the tab when outputting the
`brew linkage` declared vs. undeclared dependencies.

Fix some checks for `:clang` which should match for either `:clang` or
`:llvm_clang`. Note that's not every check.

Download refactoring

travis.yml: update xcode to 9.4

bottle_block_cop: remove

Refactor staging step for download strategies.

audit: disallow unstable specs for versioned formulae

Refactor and fix VCS download strategies.

config: handle HOMEBREW_TEMP being unset.

vendor: ruby-macho 2.0

This can happen when updating from a previous version of Homebrew.

cask audit: check for devmate and hockeyapp appcasts

 Update `Gemfile.lock` and use `Pathname#/`.