fix out-of-tree cleaning

If we build out of tree and then run `make clean`, we hit an
infinite loop where libtommath tries to enter subdirs that don't
exist and run `make clean`.

libtommath/Makefile.in

@@ -170,8 +170,8 @@ clean:
 	rm -f *.bat *.pdf *.o *.a *.obj *.lib *.exe *.dll etclib/*.o demo/demo.o test ltmtest mpitest mtest/mtest mtest/mtest.exe \
         *.idx *.toc *.log *.aux *.dvi *.lof *.ind *.ilg *.ps *.log *.s mpi.c *.da *.dyn *.dpi tommath.tex `find . -type f | grep [~] | xargs` *.lo *.la
 	rm -rf .libs
-	cd etc ; MAKE=${MAKE} ${MAKE} clean
-	cd pics ; MAKE=${MAKE} ${MAKE} clean
+	-cd etc && MAKE=${MAKE} ${MAKE} clean
+	-cd pics && MAKE=${MAKE} ${MAKE} clean
 
 #zipup the project (take that!)
 no_oops: clean