enums and ints are compatible

But like GCC do warn about changes in signedness.  The latter
leads to some changes in gen_assign_cast to not also warn about
  unsigned* = int*
(where GCC warns, but only with extra warnings).

tests/tcctest.c

@@ -591,7 +591,7 @@ void enum_test()
 {
     enum test b1;
     /* The following should give no warning */
-    int *p = &b1;
+    unsigned *p = &b1;
     printf("enum:\n%d %d %d %d %d %d\n",
            E0, E1, E2, E3, E4, E5);
     b1 = 1;