Merge pull request Homebrew#3559 from MikeMcQuaid/git-describe-1.4.0

brew.sh: latest HOMEBREW_VERSION is 1.4.0

Merge pull request Homebrew#3517 from sjackman/linkage

Implement linkage for Linux

Merge pull request Homebrew#3258 from Git-Jiro/improve_internal_stanz…

…a_command

improve 'brew cask _stanza' by checking for known stanzas

Merge pull request Homebrew#3416 from zmwangx/bump-formula-pr-fix-mes…

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bump-formula-pr: --message: add newlines before and after hr

Merge pull request Homebrew#3326 from sjackman/portable-ruby

portable-ruby: use rebuild with coverage support for Linux.

Merge pull request Homebrew#3289 from reitermarkus/cask-loader

Fix regexes in `CaskLoader`.

Merge pull request Homebrew#3211 from MikeMcQuaid/portable-ruby-coverage

portable-ruby: use rebuild with coverage support.

Merge pull request Homebrew#3182 from ilovezfs/ac_cv_xcode9

superenv: help Autotools with 10.13 SDK on 10.12

Merge pull request Homebrew#3076 from DomT4/high_sierra_b7

mac/xcode: update expected Clang on 10.13

formula_cellar_checks: remove stray backtick.