-Wall and -Werror are gcc specific compiler options and should only be
included if the compiler is gcc.  For example, the glib configure.in
handles this as follows:

--glib example start--

if test "x$GCC" = "xyes"; then
  case " $CFLAGS " in
  *[[\ \    ]]-Wall[[\ \    ]]*) ;;
  *) CFLAGS="$CFLAGS -Wall" ;;
  esac

  if test "x$enable_ansi" = "xyes"; then
    case " $CFLAGS " in
    *[[\ \  ]]-ansi[[\ \    ]]*) ;;
    *) CFLAGS="$CFLAGS -ansi" ;;
    esac

    case " $CFLAGS " in
    *[[\ \  ]]-pedantic[[\ \	]]*) ;;
    *) CFLAGS="$CFLAGS -pedantic" ;;
    esac
  fi
fi