[toplevel/libiberty] Reject configure files which warn (*BSD malloc.h bootstrap fix)

[Loren James Rittle]

>> Gerald or Loren, could you make sure this fixes your boostrap troubles on
>> FreeBSD?  You'll need autoconf 2.57 to regenerate configure.

I wrote:
> Running full bootstrap over lunch.

DJ wrote:
> This is OK if it passes the freebsd bootstraps.

Bootstrap without requiring --disable-werror into libjava, thus good
to go. - Regards, Loren