Are there any C++11 constexpr constants which can be used instead of the constant macros from <cmath>, i.e. constants like M_PI and friends? Or lacking that, any global const values which provide these constants at runtime?
M_PI is a common extension, but not part of standard C++.
Since C++20, mathematical constants such as std::numbers::pi are defined in the <numbers> header file. They are all declared constexpr.