cmake improvements

[d-callan]

inspired by #9 , but ill name the ticket somewhat generally in case other improvements make sense to do at the same time

at min we should make INSTALL_PREFIX a bit more flexible.. bioconda wants to pass a prefix rather than use whatever was set in CMakeLists.txt here, and that was a little harder to do in the bioconda recipe than id have liked. they did merge the recipe, so this ticket doesnt stand in the way of that.. but itd be nice to clean up the recipe a bit after we sort this.