rewrite fallthrough to HEIM_FALLTHROUGH to deal with new Apple SDKs

Apple clang version 14.0.0 (clang-1400.0.17.3.1) fails the build
because stds.h defines `fallthrough` as a macro which is then
expanded when base.h evaluates

  # if __has_attribute(fallthrough) && __clang_major__ >= 5

The macOS SDK defines `DISPATCH_FALLTHROUGH` as the macro instead
of `fallthrough`.

This change replaces the use of `fallthrough` in the tree with
`HEIM_FALLTHROUGH` and updates the declaration in configure logic
to define `HEIM_FALLTHROUGH` based upon existing definitions
(if any) of `fallthrough` or `DISPATCH_FALLTHROUGH`.

lib/roken/getuserinfo.c

@@ -136,7 +136,7 @@ roken_get_homedir(char *home, size_t homesz)
         }
         return home;
     }
-    fallthrough;
+    HEIM_FALLTHROUGH;
 #else
 #ifdef HAVE_GETPWNAM_R
     size_t buflen = 2048;