configure: fix detection of non-working C compiler.
(patch by Adrien Nader!)

The test tried to do something like:
  case "$foo,$bar" in
    *,*) echo 1 ;;
    *) echo "compiler not working" ;;
   esac
However the second case cannot match because of the first one.

Instead, check the return code of the command.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13844 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02