acinclude: Improve detection of not-understood compiler options with clang.

acinclude: Improve detection of not-understood compiler options with clang.

By default, clang warns about but does not fail on unknown -W options.
This made configure add the option to WARNING_FLAGS, which caused the
warning about not-understood warnings to be emitted for every file
compiled.

In combination with -Werror, clang does fail on unknown -W options.  This
commit adds -Werror during configure's warning tests, which should cause
the not-understood warnings to be detected that way.

Reported-by: Ed Maste <emaste@freebsd.org>
Tested-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>