Enable UTF char types (char16_t, char8_t etc.) when available

Currently there are defines like `BOOST_LOCALE_ENABLE_CHAR16_T` to enable `char16_t` and similar for others.

I find that a bit... difficult. We have `BOOST_NO_CHAR16_T` and similar to detect the availability automatically. So the defines are usually not required when that is used.

And the major obstacle: The library must be build with the same value (defined / not-defined) of those `BOOST_LOCALE_ENABLE_CHAR*_T` defines or the locale facets will have undefined IDs. But there is no way to know how the library was built.

Using the availability-test-macros one can assume, that if the library was built with a recent-ish compiler/standard then those types can be used.

So @artyom-beilis : Switch to using `BOOST_NO_CHAR16_T` and the like?