Fix a small silly copy-paste error.
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.

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.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>