* Fix conditional `char8_t` from `format.h` and fix `-Wunused-result` of [[no_discard]] begin() when in c++17

* Suppress `-Winconsistent-dllimport` when in clang-target-msvc

* Suppress warning _CRT_SECURE_NO_WARNINGS in MSVC and -Wdeprecated-declarations

Suppress warning _CRT_SECURE_NO_WARNINGS in MSVC and -Wdeprecated-declarations of POSIX functions in Clang target MSVC.
Those functions are used by gtest.

* Remove FMT_FUNC, mark FMT_API to export