Can't directly compare optional<int> to nullopt under GCC

The following code compiles under Clang but not under GCC:

~~~.cpp
#include
#include

BOOST_TEST_DONT_PRINT_LOG_VALUE( ::std::nullopt_t )
BOOST_TEST_DONT_PRINT_LOG_VALUE( ::std::optional )

BOOST_AUTO_TEST_CASE(y) {
BOOST_TEST( ::std::make_optional( 5 ) == ::std::nullopt );
}
~~~

I suspect that there's an issue with GCC's `std::is_constructible` needlessly explicitly instantiating `boost::test_tools::assertion::expression_base`'s `operator bool()` and I've raised [87093](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87093) about that.

However, (a) the GCC devs may well reject that issue and (b) I suspect it's workaround-able either way and it'd be really nice to be able to express this directly. Please can you look into this?

Thanks for all your work on this library.