Tutorial: Simplify logic checking for cmath functions

Since commit 07223c5c27 (Tutorial: Update Step 5 to work on Windows,
2020-02-18, v3.18.0-rc1~655^2) the logic does not work on non-Windows
platforms when cmake is re-run on an existing build tree.  It is also
more complicated than we'd like for a tutorial example.  Avoid the need
to consider the `m` library case by performing the check as C++.

Since `check_cxx_symbol_exists` cannot handle overloaded functions
like `exp` and `log`, check with `check_cxx_source_compiles` instead.
This also presents a more general-purpose example in the tutorial.

Fixes: #23524

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