I added AM_MAINTAINER_MODE_DEFINES to gnome-common.m4.

This macro checks maintainer mode, and if it's on defines two make
variables:

DISABLE_DEPRECATED="-DG_DISABLE_DEPRECATED ..."
ENABLE_WERROR="-Werror"

If maintainer mode is off, it sets both variables to ""

The idea is that we shouldn't ship packages with DISABLE_DEPRECATED in
them.  If we do, then our packages may break when new versions of things
come out.  However, DISABLE_DEPRECATED should be on for developers when
working from cvs.  This is also true in many cases for -Werror
(depending on the project.)

Thanks,
    Chris