build: add --enable-werror

This allows enabling -Werror in a consistent way.  Note that this is
different from just specifiying it in CFLAGS, since that would either
break configure tests (if done on ./configure), or would override
configure's CFLAGS (if done on make).

Using --enable-werror instead provides a new WERROR variable that is
additionally used during make with a consistent set of warning flags.

The tests/ directory is exempt.  (Rationale being, better to have more
tests than pedantically complain about them.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>