Fix shared library building failure on Windows with non MSVC

For example, now we can built shared library on Window with MinGW.

This changes improve features added in #1467.

Add support for custom filename calculator in rotating_file_sink.

Add support for FMT_STRING compile time checking.

Address code review comments: revert perfect forwarding on places that didn't need it, remove negative compilation unit test.

Add support for FMT_STRING compile time checking. Add negative compilation unit test for compilers and c++ standard that support relaxed constexpr.

fix deadlock on filename() call. Thanks @IIFE 

Fixed ringbuffer_sink last_raw and last_formatted

Fixed ringbuffer_sink last_raw and last_formatted giving the first lim number of elements instead of last.