configure: Set OS/2 objformat to a.out.

Signed-off-by: Diego Biurrun <diego@biurrun.de>

configure

@@ -2473,6 +2473,7 @@ case $target_os in
         ;;
     os/2*)
         ln_s="cp -f"
+        objformat="aout"
         add_ldflags -Zomf -Zbin-files -Zargs-wild -Zmap
         SHFLAGS='$(SUBDIR)$(NAME).def -Zdll -Zomf'
         FFSERVERLDFLAGS=""