Fix src/Makefile being wrongly overwritten by src/sman/Makefile

[binghe]

In src/Makefile.pamphlet, the following chunk of code extracts another Makefile from bookvol6.pamphlet into src/sman and execute the new Makefile for building some executions (spadclient, session, etc.):

\begin{chunk}{smandir}
smandir: ${SPD}/books/bookvol6.pamphlet
	@echo 5 making ${SRC}/sman
	@( cd sman ; \
	   ${BOOKS}/tanglec ${BOOKS}/bookvol6.pamphlet >Makefile ; \
	   ${ENV} ${MAKE} )

\end{chunk}

Unfortunately, the command cd sman failed, because the sub-directory src/sman doesn't exist. But this is not a fetal issue, as the next shell commands still successfully extracted the new Makefile and execute it, just it is directly put into src/ and overwrote the existing one. The final resulting Axion build didn't miss anything which should be built but not, just the toplevel platform-specific Makefile.SYS now becomes invalid, because the calling src/Makefile is not the correct one. A simple solution is to do mkdir -p sman before cd sman.