Autodetect the best option to use Boost.Test

The conditional inclusion of either the library or the header-only
version of the Boost.Test header wasn't tremendously useful in practice
because the tests/CMakeLists.txt file would unconditionally add compile
definitions to basically fore dynamic linking.

This patch adds feature detection to the tests/CMakeLists.txt file to
determine whether to use dynamic linking, static linking or the
header-only version (in that order of preference, for best performance).

The automatic detection could be overridden if needed by defining the
TOML11_WITH_BOOST_TEST_{HEADER,STATIC,DYNAMIC}
variables on the CMake command line.

While we are at it, instead of having a conditional
#define BOOST_TEST_NO_LIB
in each unit test file, handle this once in the CMakeLists.txt file.

tests/test_find.cpp

@@ -3,7 +3,6 @@
 #ifdef UNITTEST_FRAMEWORK_LIBRARY_EXIST
 #include <boost/test/unit_test.hpp>
 #else
-#define BOOST_TEST_NO_LIB
 #include <boost/test/included/unit_test.hpp>
 #endif