Detection of availability of 128-bit integers should be left on Boost.Config

[Petr Machata <pmachata@…>]

The following code in lexical_cast.hpp appears wrong:

#if (defined(BOOST_LCAST_HAS_INT128) && !defined(__GNUC__)) || GCC_VERSION > 40700
#define BOOST_LCAST_HAS_INT128
#endif

I'm uncertain whether BOOST_LCAST_HAS_INT128 serves some purpose such as per-library configuration. It seems to me it would be best to simply defer this logic to config.hpp, drop this piece of code, and later on dispatch on BOOST_HAS_INT128. I'm attaching a patch to this effect.

Note that this logic currently breaks on i386, where GCC doesn't support 128-bit data types even though it otherwise satisfies the above condition.

If you wish to have this configured specifically for lexical cast, I can work in some logic that overrides BOOST_HAS_INT128 in favor of whatever BOOST_LCAST_HAS_INT128 setting came from the "outside" (if any).

[Petr Machata <pmachata@…>]

A fix.

[Petr Machata <pmachata@…>]

Oops, and I see there's a duplicate of this bug already: #8790.