default case for hostDynamicLibExt.

this is useful for e.g. checking things out in ghci, where we don't have
the fancy environment variables from config.mk.
Added .ghci specifying language extensions.

Seems like -XFlexibleInstances is expected everywhere; I just copied the
four extensions specified in idris.cabal for completeness' sake.
Merge pull request #414 from startling/for-ghci

Made Idris a little more ghci-friendly.