dnl
dnl JH_ADD_CFLAG(FLAG)
dnl checks whether the C compiler supports the given flag, and if so, adds
dnl it to $CFLAGS.  If the flag is already present in the list, then the
dnl check is not performed.
AC_DEFUN([JH_ADD_CFLAG],
[
case " $CFLAGS " in
*@<:@\	\ @:>@$1@<:@\	\ @:>@*)
  ;;
*)
  save_CFLAGS="$CFLAGS"
  CFLAGS="$CFLAGS $1"
  AC_MSG_CHECKING([whether [$]CC understands $1])
  AC_TRY_COMPILE([], [], [jh_has_option=yes], [jh_has_option=no])
  AC_MSG_RESULT($jh_has_option)
  if test $jh_has_option = no; then
    CFLAGS="$save_CFLAGS"
  fi
  ;;
esac])