Fix make clobber in 2015/yang + update .gitignore

Remove output and output.c.

2015/yang/.gitignore

@@ -4,6 +4,8 @@
 indent
 indent.c
 indent.o
+output
+output.c
 prog
 prog_c11
 prog_c90

2015/yang/Makefile

@@ -230,7 +230,7 @@ clean:
 	fi
 
 clobber: clean
-	${RM} -f ${TARGET} ${ALT_TARGET}
+	${RM} -f ${TARGET} ${ALT_TARGET} output output.c
 	${RM} -rf *.dSYM
 	@-if [ -e sandwich ]; then \
 	    ${RM} -f sandwich; \