Fix build with Apple clang & C++17 (bessel functions not defined)

[mikeperri]

The cyl_bessel_i function isn't defined if you're building with clang, but MathHelpers assumes it is as long as you're using C++17. So we need to add a check for that feature.

I tested this on an Intel Mac. CMake identifies the compiler as "AppleClang 12.0.5.12050022."

I added

SET(CMAKE_CXX_STANDARD 17)
SET(CMAKE_CXX_STANDARD_REQUIRED ON)

to the root CMakeLists.txt, and I ran

cmake -DCMAKE_BUILD_TYPE=Release .
make -j$(sysctl -n hw.ncpu)