formula: help CMake with 10.12 SDK on 10.11

Use git describe to get the HOMEBREW_VERSION.

update.sh: don't print "checkout branch" message.

This adds a CMake cache entry to std_cmake_args specifying that the
function clock_gettime is not available on 10.11 in order to avoid
runtime errors such as

  dyld: lazy symbol binding failed: Symbol not found: _clock_gettime

when the build system is confused by Xcode 8's weak symbols.

Other weak symbols on 10.11, which may merit the same treatment in the
future, can be found with

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

Ask for `sudo` when moving Caskroom.

It’ll only get printed for people getting updated to tags now and these
are people who haven’t run a `dev-cmd` so we want to air on the side of
telling them less stuff that will confuse them and assume people who
have manually made another `git` branch will know how to get back to it.

For tagged commits produces the output:
- `1.0.1`

For untagged commits with a dirty tree produces the output:
- `1.0.1-19-g23efbc5-dirty`

Performance:
```
git describe --tags --dirty 2> /dev/null
0.07s user 0.01s system 96% cpu 0.086 total
```

This means we can tag any commit without needing to manually remember
to bump the revision every time.

Delete old cctools-based relocation code.

uninstall: improve pronoun for multiple version message

sandbox: tweaks