Check*CompilerFlag: Do not set result as a normal variable too

This was previously fixed by commit d4659091 (Check*CompilerFlag: Do
not set result as a normal variable too, 2020-09-21, v3.18.3~1^2^2), but
was regressed by refactoring in commit 90dead02 (CheckCompilerFlag:
unified way to check compiler flags per language, 2020-09-25,
v3.19.0-rc1~88^2) due to the changes being developed concurrently.
Fix it again, and add a test case.

Fixes: #21207