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/hx509/cert.c

@@ -2441,10 +2441,10 @@ hx509_verify_path(hx509_context context,
 		 * EE checking below.
 		 */
 		type = EE_CERT;
-                fallthrough;
+                HEIM_FALLTHROUGH;
 	    }
 	}
-        fallthrough;
+        HEIM_FALLTHROUGH;
 	case EE_CERT:
 	    /*
 	     * If there where any proxy certificates in the chain