Fix import std in clang++

[shyeyian]

When #define FMT_IMPORT_STD, {fmt} will do import std instead of #include <std...>. However this is currently broken on clang++. This PR fixes:

1. clang++ cannot find memcpy, memcmp, abort when importing std. Fix it by adding #include <string.h> and #include <stdlib.h> in global module.
2. include/fmt/std.h uses std::__bit_const_reference which is not exported by libc++ and cannot be found. Guard it by macros.

BTW, we can import C std symbols by import std.compat, and in this case we do not need the <limits.h>, <stdint.h>... as well. So that I add an optional FMT_IMPORT_STD_COMPAT macro to tell {fmt} whether import std.compat.

Thank you!

[vitaut]

Thanks for the PR. The fixes LGTM but I don't think we need to add another configuration macro (FMT_IMPORT_STD_COMPAT).

[shyeyian]

Thanks for the PR. The fixes LGTM but I don't think we need to add another configuration macro (FMT_IMPORT_STD_COMPAT).

Okay, I've reverted that :)

[vitaut]

Merged, thanks!