configure: fix as/aspp for FreeBSD (works for both gcc and clang)

(Patch by Andrew Ray)

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14727 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
-o sets the module name

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14728 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02