Utilities/Sphinx: Add role and directive for 'envvar' in CMake domain

This enables cross-reference syntax for CMake environment variables:

    :envvar:`SOMEVAR`

and definition of CMake environment variables via a directive:

    .. envvar:: SOMEVAR

It also adds environment variables defined by the directive and by
`Help/envvar/SOMEVAR.rst` documents to the index.

This `envvar` role and directive is defined in our `cmake` domain
and overrides the equivalent `envvar` role and directive provided
by Sphinx in its default domain.  This is okay because we build
CMake documents in the `cmakd` domain.

This follows up the work from commit v3.10.0-rc1~43^2 (Help: Document
CMake's environment variables, 2017-09-01) that originally added
`envvar` documentation.

0 files changed, 0 insertions, 0 deletions