Shut up some warnings in the robdd directory.

robdd/bryant.c:
    Add a cast to shut up a gcc warning.

robdd/Makefile:
    Define a make variable as empty if it is undefined.

robdd/Makefile

@@ -82,6 +82,10 @@ RSET=$(DEF)RESTRICT_SET
 
 # Main makefile
 
+ifeq ($(origin EXTRAOPTS),undefined)
+EXTRAOPTS =
+endif
+
 CFLAGS=$(CHOSENOPTIM) $(DEF)$(WHICH) $(OPTS) $(RSET) $(QOPT) $(MSGS) $(EXTRAOPTS)
 
 all: $(TESTS)

robdd/bryant.c

@@ -1115,7 +1115,7 @@ MR_ROBDD_prev_nonelement(MR_ROBDD_bitset *set, MR_ROBDD_int *var,
             }
         } while ((bits=*--ptr) == ~0);
         vr = (wd<<MR_ROBDD_LOG_BITS_PER_WORD) + MR_ROBDD_BITS_PER_WORD-1;
-        msk = 1<<(MR_ROBDD_BITS_PER_WORD-1);
+        msk = ((MR_ROBDD_bitmask) 1)<<(MR_ROBDD_BITS_PER_WORD-1);
     }
     /* I know there's an earlier bit clear in bits, so this is safe */
     while ((bits&msk) != 0) {