Simplify per @tarjoilija's feedback
Simplify per @tarjoilija's feedback

Use source in `${ZGEN_INIT}` instead of -zgen-source. Fixes #7, with the
welcome side effect of being 11% faster.

unixorn/zgen,outcoldman/zgen,m42e/zgen,yous/zgen,forivall/zgen,tarjoilija/zgen,ixti/zgen,N4M3Z/zgen,sevanteri/zgen