Add Configuration via CMake and replace all detection macros with BEMAN_ macros

[mborland]

As requested by @ednolan

[mborland]

Once this one is merged, I'll tag a release so that we can add vcpkg and Conan support

[mborland]

Failures are unrelated to the changes

[eisenwave]

This design is clever, but it's not yet complete.

We still have compiler identification that's not covered here. Isn't it possible to install something built with GCC and to then get a different outcome when building that installation with Clang?

Another problem is that lots of our feature detection bypasses the standard feature-test macros entirely by using __has_builtin instead. So things like #if BEMAN_BIG_INT_HAS_BUILTIN(__builtin_add_overflow) presumably violate the Beman standard by doing flag forking as well; these are just compiler version detections under the hood. I don't actually mind merging this now and incrementally fixing the rest of the flag forking though.

[mborland]

This design is clever, but it's not yet complete.

We still have compiler identification that's not covered here. Isn't it possible to install something built with GCC and to then get a different outcome when building that installation with Clang?

Theoretically as no version of Clang supports say <stdfloat> while libstdc++ has since 13.

Another problem is that lots of our feature detection bypasses the standard feature-test macros entirely by using __has_builtin instead. So things like #if BEMAN_BIG_INT_HAS_BUILTIN(__builtin_add_overflow) presumably violate the Beman standard by doing flag forking as well; these are just compiler version detections under the hood. I don't actually mind merging this now and incrementally fixing the rest of the flag forking though.

I can pull all of those into config before merge.

[eisenwave]

It's probably best to rip off the bandaid once and fix all the flag forking, including the __has_builtin stuff in this PR.

I assume that's pretty straight forward. The cleanest solution may be to convert code like

#if BEMAN_BIG_INT_HAS_BUILTIN(__builtin_add_overflow)
  __builtin_add_overflow(x, y)
#endif

to

#ifdef BEMAN_BIG_INT_BUILTIN_ADD_OVERFLOW
  BEMAN_BIG_INT_BUILTIN_ADD_OVERFLOW(x, y)
#endif

where BEMAN_BIG_INT_BUILTIN_ADD_OVERFLOW sits in the #else part in config.hpp as well and ends up defined as

#if BEMAN_BIG_INT_HAS_BUILTIN(__builtin_add_overflow)
  #define BEMAN_BIG_INT_BUILTIN_ADD_OVERFLOW(...) __builtin_add_overflow(__VA_ARGS__)
#endif