Check*: Restore support for arbitrary result variable names

In commit db76876db5 (Modules: Use new SOURCES_FROM_* try_compile (1/2),
2022-09-26, v3.25.0-rc1~74^2~1) and commit 41f7b1169a (Modules: Use new
SOURCES_FROM_* try_compile (2/2), 2022-09-26, v3.25.0-rc1~74^2) the
switch to `SOURCE_FROM_*` required a stronger precondition (the second
argument to check_include_files must not have path components) than
before (any variable name could be used).

Fix that by transforming the variable name to a C identifier before
feeding it to try_compile as a filename.  The filename is unspecified by
the documentation, and the file itself is only temporary, so that should
work fine.

I have gone through all the occurrences of `SOURCE_FROM_*`, and
identified these two that require changes.  The rest should work fine as
the filenames do not depend on input variable names.

Fixes: #24204

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