Disambiguate fmt logging methods that are using variadic templates.

As variadic template arguments can be zero length, we need to specify that at least one fmt argument is provided, to distinguish these methods from the existing trivial method that takes no fmt arguments.

Without this, static analysers such as ReSharper flag the logging calls as errors.

Update README.md

fmt: update bundled fmt to 3.0.1 (7fa8f8f)

Signed-off-by: Damien Zammit <damien@zamaudio.com>

Use double-braces in std::array initialization.

Don't hardcode '.txt.' log file name suffix (resolve #333)

Signed-off-by: Ľubomír Carik <Lubomir.Carik@gmail.com>

Replace static global std::string arrays by Meyer singletons. 

Required for C++11 support

Add prefix for BUILD_TESTING cmake option

This is helpful when using spdlog as a dependency (git submodule) when a
master project is not interested in spdlog tests. Using
"BUILD_TESTING" name may create a confusion.
Extra: BUILD_EXAMPLE variable already have a prefix.

removed empty lines

option to  prevent child processes from inheriting log file desciptors (#define SPDLOG_PREVENT_CHILD_FD)

Use the feature checking macros to detect thread_local support in clang.