ABI-tag __has_include-guarded differing code

[tzlaine]

From David Sankel and me, from the Boost review:

The usage of has_include is a ODR nightmare for large codebases. It would be better to generate some kind of config.hpp that sets these definitions at compile time.

This is a great point.  There are three places where this currently happens:

1) Boost.TypeIndex vs typeinfo (for printing type names in the trace) 2) BOOST_ASSERT vs. assert 3) Spirit X3 vs. charconv vs. Boost.Charconv

1 and 3 are easily addressed by adding an extra template parameter to the few function templates that use those APIs, so a mismatch will be a build break, not ODR. 2 does not apply to production code, and any use of BOOST_ASSERT already has the same problem anyway.

[tzlaine]

The technique I used for this was not explicit ABI-tagging (using the GCC+Clang abi_tag attribute), because there does not appear to be an MSVC equivalent. Instead, I put each alternate implementation in its own inline namespace.