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.
Actually, since `floating` is used for toml::types, `as_floating`
seems to be clearer. But currently `is_*` functions uses `float`,
not `floating`, so `as_float` is chosen for the consistency.
In a future release, possibly v3, those names may need to be
re-considered for clarity.
