Makefile.am: Don't use dots in -osuf

This fixes
  ./configure --enable-haskell-tests && make hs-tests
builds on GHC 7.8.

It changes `-osuf .ext` to `-osuf ext`; the old form was allowed in
GHC <= 7.6, but GHC 7.8 seems to have problems with it:
  https://ghc.haskell.org/trac/ghc/ticket/9760

The form without dot works in all GHC versions, and we don't have
dots in other -osuf arguments in the same file either, so this
was probably an accident.

Signed-off-by: Niklas Hambuechen <niklash@google.com>