[It does](https://github.com/adymo/homebrew-kde/blob/1477b032603a956e0c55294a5bd09e8c43b1f7a2/base_kde_formula.rb#L47) and defaults to `/usr/local`. This puts it in `$(brew --prefix)/Cellar/marble/x.x.x`.

Also note in Marble's INSTALL file:

> 4. Known Issues:
>
> - If you don't specify any install path using cmake then Marble will probably install under /usr/local.