Prevent gratuitous Automake failures.

Prevent gratuitous Automake failures.

Automake-1.12 introduces some new warnings, as well as fixing an earlier
bug that prevented -Wall from enabling all warnings.  Since we were
using -Werror, this causes Automake to fail.

However, the warnings are gratuitous: some are warning about stuff
provided by libtool (which should be fixed in libtool, not worked
around in this package), and the others are warning about the non-
portability of (hopefully!) portable constructs.  Most are automake
bug #9587 <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=9587>, but
Automake also warns about the $(.FEATURES) trick to use order-only
dependencies when available.