add -fwarn-dodgy-foreign-imports (see #1357)

add -fwarn-dodgy-foreign-imports (see #1357)

From the entry in the User's guide:

-fwarn-dodgy-foreign-imports causes a warning to be emitted for
foreign imports of the following form:

foreign import "f" f :: FunPtr t

on the grounds that it probably should be

foreign import "&f" f :: FunPtr t

The first form declares that `f` is a (pure) C function that takes no
arguments and returns a pointer to a C function with type `t`, whereas
the second form declares that `f` itself is a C function with type
`t`.  The first declaration is usually a mistake, and one that is hard
to debug because it results in a crash, hence this warning.