Make sure we can build the manual when starting from a clean pull.
While here, build the manual by doing:
cd sbcl/doc && sh make-doc.sh
cd sbcl/doc/manual && make
as on some platforms, `make' is not the right make (using $MAKE would also
work with most makes). I know the page doesn't have to build everywhere
that SBCL itself does, but this was low-hanging fruit.