Add a CMake option to use a header-only external fmt

Update README.md

In order to use basic_logger_mt an additional include is required.

Fix unexpected log macro expansion.

Namespace qualify stat functions

Add spdlog::initialize_logger

Add systemd sink.

Expose should_do_colors_ in ansicolor_sink.h

Add tweak support for user short level names

Fix typo in Android logger

Add CMake option for conditional installation

Remove unnecessary log function

Allow custom formatter in sink's ctor

Allow the inclusion of an external fmtlib as as sub_directory

Shadowed parameter

However, not in bundled fmt. Corrects compiling with basic logger with header only and -Werror.