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