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/snprintf.c

@@ -515,7 +515,7 @@ xyzprintf (struct snprintf_state *state, const char *char_format, va_list ap)
 	    }
 	    case '\0' :
 		--format;
-                fallthrough;
+                HEIM_FALLTHROUGH;
 	    case '%' :
 		(*state->append_char)(state, c);
 		++len;