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.
2 files changed
