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"

cmake: misc improvements

These checks come from [1]. The `MATCHES` operator is used for clang
because of Apple's special string.

[1] http://stackoverflow.com/questions/10046114/in-cmake-how-can-i-test-if-the-compiler-is-clang/10055571#10055571

As per the docs [1], there's a standard variable for this purpose. This
introduces a behavior change, the tests are now being built by default.

[1] https://cmake.org/cmake/help/v3.0/module/CTest.html

This is a usual CMake way of ensuring that IDEs have a way of showing
all source files which comprise this header-only library. It works in
the Qt Creator, for example.

Compatibility with Oracle Developer Studio 12.5 on  Solaris

Fix Klockwork compilation warning

Fix compilation error C2664  on VS2013

1. Removing "return" from void functions.
2. Using "const" for operator= argument.

No converting constructor