fix `initializer element is not constant` error

I faced this error on an (old) 10.6 Mac OS X equipped with GCC 4.2.1 - clang 3.0 does not complain.

It sounds as if only [*literal constants*](http://stackoverflow.com/a/3025106/1688185) can be used (in C) for such assigments. I found this [suggestion](http://stackoverflow.com/a/795189/1688185) to work with `enum`-s instead of macros pretty useful.