This ports the minimal Jambase to native C++. This removes the need for
mkjambase and the Jambase files. To accomplish that it adds C++
utility wrapers around Jam language primitives. Which makes future
similar work much easier.
Some system headers on POSIX systems indirectly include strings.h in extern "C"
region. This sometimes results in Boost.Build's strings.h being included into
such region, which marks all string_* functions as extern "C" and changes their
name mangling rules accordingly, which causes linking errors. To resolve this
header conflict, this commit renames strings.h to jam_strings.h. And strings.cpp
to jam_strings.cpp for consistency.
Fixes https://github.com/boostorg/build/issues/468.
