A few releases back, I changed things so that lein-cljsbuild would remove the intermediate JS source directory before each compiler run, due to numerous weird bug reports that all went away after a "cljsbuild clean" was run. This was a defensive maneuver, to hide the underlying problem (for which I still don't understand the cause).
At this point, though, I think I should make the "auto-clean" behavior an option, with a default value of "off". It slows things down considerably, and it may not even be necessary anymore. Probably, if problems do show up again, we should track down the root cause and fix that (painful as it may be).
Build incrementally by default, for speed.
A new :incremental option has been introduced to allow the old behavior to be
selected, if this breaks things for anyone. Closes #94.