build: remove --disable-Werror from AM_DISTCHECK_CONFIGURE_FLAGS

It must be possible to compile a GNOME module from a tarball without
--disable-Werror, if the module produces warnings when compiling the
source code (which is currently the case for Devhelp, there are warnings
about the use of deprecated functions).

https://bugzilla.gnome.org/show_bug.cgi?id=779378