Fix build by always disabling -Werror and release 0.2019.0-0co1

I've pushed the v0.2019.0 and apertis/0.2019.0-0co1 tags already, so Jenkins should be happy when merging this.

Once landed in OBS, we'll need to fast-forward the v2019 branch as well and copypac on OBS.

configure.ac: Fix non-release builds by always disabling -Werror

Non-release builds are currently failing due to some deprecations in appstream-glib 0.6.9, released in February 2017:

  CC       src/update-component-index.o
src/update-component-index.c: In function ‘main’:
src/update-component-index.c:250:11: error: ‘as_app_set_source_file’ is deprecated: Use 'as_app_add_format' instead [-Werror=deprecated-declarations]
           as_app_set_source_file (fallback_app, top);
           ^~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/libappstream-glib/appstream-glib.h:27,
                 from src/update-component-index.c:16:
/usr/include/libappstream-glib/as-app.h:753:8: note: declared here
 void   as_app_set_source_file  (AsApp  *app,
        ^~~~~~~~~~~~~~~~~~~~~~
src/update-component-index.c:251:11: error: ‘as_app_set_source_kind’ is deprecated: Use 'as_format_set_kind' instead [-Werror=deprecated-declarations]
           as_app_set_source_kind (fallback_app, AS_APP_SOURCE_KIND_UNKNOWN);
           ^~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/libappstream-glib/appstream-glib.h:27,
                 from src/update-component-index.c:16:
/usr/include/libappstream-glib/as-app.h:761:8: note: declared here
 void   as_app_set_source_kind  (AsApp  *app,
        ^~~~~~~~~~~~~~~~~~~~~~
src/update-component-index.c:329:7: error: ‘as_app_get_source_file’ is deprecated: Use 'as_app_get_formats' instead [-Werror=deprecated-declarations]
       source = as_app_get_source_file (app);
       ^~~~~~
In file included from /usr/include/libappstream-glib/appstream-glib.h:27,
                 from src/update-component-index.c:16:
/usr/include/libappstream-glib/as-app.h:756:14: note: declared here
 const gchar *as_app_get_source_file  (AsApp  *app)
              ^~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[1]: *** [Makefile:3580: src/update-component-index.o] Error 1

Since Canterbury is no longer under active development, keep the warnings but make them non-fatal by forcing the IS-RELEASE parameter of the AX_COMPILER_FLAGS() macro to yes, instead of being derived from the value set automatically by the AX_IS_RELEASE() macro.