includePaths is already modified in `setGhcjsPlatform'

setGhcjsPlatform calls addPlatformDefines which, among a few other
things sets up the `includePaths` dynflag
Introducing runGhcjsSession

Mainly introducing new functions in the GHCJS module,
still have to figure out where some those should really go.
Merge branch 'master' into cleanup
merge branch 'master'
simpler versino of the
Merge branch 'master' into cleanup
Merge branch 'master' of https://github.com/ghcjs/ghcjs into cleanup
Merge branch 'cleanup'