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"

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.