Merge topic 'enable_language-deduplicate'

e3c8012ccd Help: Document enable_language accepting multiple languages
b4fd385c9b cmMakefile: Dedupe languages when enabling them
66bfe14309 cmMakefile: Refactor parameter and variable names for EnableLanguage

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Merge-request: !7426