cabal: add threaded flag
cabal: add threaded flag

To help people compiling Bustle on mips, s390 and sparc, where the
threaded runtime isn't available, here's a flag which lets you turn it
off. I don't think Bustle strictly needs it (I don't think it uses any
Haskell-land threads) but equally I don't see any particular reason not
to use it on platforms where it's available.

wjt/bustle,wjt/bustle