Better standalone error message?

[NAThompson]

When using the standalone, I get:

In file included from ./include/boost/multiprecision/mpfr.hpp:9:
In file included from ./include/boost/multiprecision/number.hpp:10:
./include/boost/multiprecision/detail/standalone_config.hpp:15:10: fatal error: 'boost/config.hpp' file not found
#include <boost/config.hpp>
         ^~~~~~~~~~~~~~~~~~
1 error generated.

This is an expected error:

// Boost.Config is dependency free so it is considered a requirement to use Boost.Multiprecision in standalone mode
#include <boost/config.hpp>

but could it give a nice error message as to why it's not supported using __has_include?

[Lastique]

I think this needs to be moved to Boost.Multiprecision.