enable_language: Fix test for working compiler with CMP0126 NEW behavior
Update the logic that converts a
try_compile result from a cache
entry to a normal variable to tolerate an existing normal variable
under CMP0126 NEW behavior. Otherwise the
is ignored because CMake uses the false value of the normal variable,
and CMake incorrectly reports that the compiler does not work.
This went unnoticed for some languages (e.g. C and CXX) because the check for a working compiler is skipped if ABI detection works. It does affect other languages (e.g. CSharp).
Fixes: #22423 (closed)