tcp_client.h bugfix for macOS and add tcp example 

-> fix the issue that the head is not compilable bc. of missing declaration
-> optimize if/else block in connection function. Else not needed bc. of break in if

Fix for issue #1627: Global get_level() and should_log()

* Added: `spdlog::get_level()` API function - like `logger::level()`, except for the name change
* Added: `spdlog::should_log()` API function - like `logger: should_log()`

Bump CMake requirement to 3.10, don't enable C language

spdlog doesn't seem to actually require the C language.
Not enabling it results in a significant initial configure time speedup.

* Reduces the range of possible version we'd need to test with.
* Enables newer policies reducing possible deprecation warnings from new policies.
* Allows removing some code for compatibility with older versions.
* Coincides with LLVM's bump to requiring CMake 3.13.

fmt 7.0.0 renamed the internal namespace to detail.

See: https://github.com/fmtlib/fmt/issues/1538

Cleanup options-related lines in CMakeLists.txt

Twice tolerance waiting time with MinGW

Fix shared library building failure on Windows with non MSVC

Increased tolerance waiting time for MinGW in dequeue-empty-nowait test
case, because this test case not passed on slow and powerful computers.

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