Check*: Tolerate variables set with names of languages
Fix the language checks added by commit 90dead02 (CheckCompilerFlag: unified way to check compiler flags per language, 2020-09-25, v3.19.0-rc1~88^2), commit 10ae907d (CheckSoureCompiles: Add a unified way to check if a source compiles, 2020-09-14, v3.19.0-rc1~118^2~1), and commit 357e2ef4 (CheckSoureRuns: Add a unified way to check if a source runs, 2020-09-14, v3.19.0-rc1~118^2) to work when variables of the language names are set. Fixes: #21500