Conditional compilation actually based on the target, not just presence of cljs

Support for a fix to https://github.com/Prismatic/plumbing/issues/42

Not sure why this works in our projects but not in the example given there, but it seems that we were sometimes generating Clojurescript when macroexpanding into Clojure (possibly depending on compilation order).  This fixes things using the strategy described here:

https://groups.google.com/d/msg/clojurescript/iBY5HaQda4A/w1lAQi9_AwsJ