In commit 80f5d28813 (Tutorial: Update step 2 style, 2022-07-25,
v3.25.0-rc1~226^2) we replaced some uses of `USE_MYMATH` with `MY_MATH`.
Restore the former name for consistency with the rest of the tutorial.
* Update minimum required version to 3.10
* Use VERSION argument to project command rather than separate variables
* Replace `endif(USE_MYMATH)` with more modern `endif()`
* Simplify the call to 'configure_file()'
* Add comments to tutorial.cxx to use as anchors in documentation
* Remove CMakeLists and TutorialConfig.h.in files that users should
create. Consequently, remove Step1 from CMake tests.
Migrate tutorial text from individual `directions.txt` files to the main
tutorial document. Add some comments to source code to provide anchors
for inclusion.
