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.

fixed compilation error in clang before version 8

Fix compilation errors in "wincolor_sink.h"