Xcode: fix version detection with unknown clang

Use more descriptive heredoc names.

Fix `SystemCommand` `:path`.

diagnostic: be clearer about what we support.

And be even more explicit whose problem it is when things are broken.

Bump github-pages from 187 to 188 in /docs

global: handle HOMEBREW_BOTTLE_DOMAIN fallback.

Simplify and fix external `brew cask` command detection.

Fix PKG installer environment

Bumps [github-pages](https://github.com/github/pages-gem) from 187 to 188.
- [Release notes](https://github.com/github/pages-gem/releases)
- [Commits](https://github.com/github/pages-gem/compare/v187...v188

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

If users have auto-updated then these variables may not be set.

language/python, docs: updates for python 3.7 

tab: if parsing JSON fails output the filename.

link: refactor, reorder and fix bugs.

The change in #4441 broke the handling of the `elsif`s due to the
change in logic. As every block here has a `next` there's no need to do
an `elsif` in here at all. Additionally, reorder the conditions in here
so you get an appropriate message depending on what you're trying to do.
Finally, tweak some of the messaging to remove things that are ignored
and tell people correct commands to run to link things.

This should make it easier to debug and resolve things like #4439.

link: fix undefined `dep_f`

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

This commit solves an issue where the environment handed to
`/usr/sbin/installer` is not the same as the environment used by the
graphical PKG installer.

This is evident in some post-install scripts, e. g. the
`component-10.pkg/Scripts/postinstall` script in the `dymo-label`
cask. The code says:

```
USER_ID=`id -u ${USER}`

launchctl bootstrap gui/$USER_ID /Library/LaunchAgents/com.dymo.dls.webservice.plist
```

The graphical installer will export e. g. `USER=alice`, and
everything works as intended.

However, `brew cask install` does not override `sudo`’s default,
which is `USER=ROOT`. This violates the assumptions in the script.

This commit fixes the issue by configuring `sudo` to override the
following environment variables with the proper user name:

- `LOGNAME`
- `USER`
- `USERNAME`

linkage: fix output of optional dependencies.