Perform sed and -mt code checks in C++ mode

The default language is C, but since Boost is inherently a C++ library, it makes sense to invoke the C++ compiler for all Boost-related Autoconf tests, even if no C++ code is actually being compiled.

This change wraps the call to `_BOOST_SED_CPP` when detecting Boost's version, as well as the code for detecting the environment's multithreading status in `_BOOST_GUESS_WHETHER_TO_USE_MT`.

This has an added side effect, which is that when not using Libtool, there's no dependency on C at all — the `CC`, `CFLAGS`, and `CPP` macros don't affect the configure script. Technically, Boost.m4 depends on Libtool, but in my experience, the only thing it uses is the `$libext` variable, and then only when searching for a static library to link with. I can circumvent the full Libtool dependency by simply setting `$libext` directly (usually to `a`, but to `lib` on Windows compilers). This allows for faster configurations since we don't have to detect and test the capabilities of a C compiler that will never actually be used.

Even when Libtool *is* used (as recommended), I think it's still better to perform the version and multithreading checks in C++ mode since they should really be testing C++ headers and C++ compiler settings, even when the analogous C values would most likely be the same.