sha2 headers: remove ifdefs dealing with pre-C99 junk
make_config: use a macro for exports
support new variable B for builddir

it may be convenient to point the variable $S aka pkgroot
to an existing sabotage directory, but have the build dir in
another place.
the new variable B allows to specify the place for the builds,
and, if unset, points to $S/pkg, so the old behaviour stays intact.