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 by undefining the internal _GLIBCXX_USE_STD_SPEC_FUNCS macro, making <cmath> roughly the same size as on C++11. If you need the additions, #include <cmath> before this header.

The C++17 additions are also present on MSVC STL since version 2017 15.7, however there the additions are very lightweight, so no workaround is needed. For Clang, at the time of writing, libc++ 10 doesn't have these additions yet. On these two this header effectively does just a simple #include <cmath>.