Merge topic 'doc-xref-envvar'

63e0c16e3d Help: Link from CMAKE_<LANG>_FLAGS to <LANG>FLAGS environment variables
8acf46caf1 Utilities/Sphinx: Add role and directive for 'envvar' in CMake domain
7e532abc78 Utilities/Sphinx: Fix QtHelp generator identifiers

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Robert Maynard <robert.maynard@kitware.com>
Merge-request: !1985