Corrade/Utility/StlMath.h file new in 2019.10

Include <cmath>, but without the heavy C++17 additions.

With libstdc++ from GCC 6 and newer, #include <cmath> is above 10k lines when building with -std=c++17. This is due to C++17 additions to the math library, such as std::riemann_zeta(). Because these APIs are seldom used in graphics-related tasks, it doesn't make sense to have compile times inflated by them. If you include Corrade/Utility/StlMath.h instead, it will ensure the new functions are not present, making the <cmath> roughly the same size as on C++11. If you need the additions, #include <cmath> before this header.

Currently, the C++17 additions are present neither on libc++ 8 nor MSVC STL 2017, so there this header effectively does just a simple #include <cmath>.