My build fails with this message:

'g_array_index' instead [-Werror=deprecated-declarations]
mv -f .deps/libnm_util_la-nm-setting-bluetooth.Tpo .deps/libnm_util_la-nm-setting-bluetooth.Plo
nm-param-spec-specialized.c:471:2: error: 'g_value_array_get_nth' is deprecated (declared at /home/wena/output/include/glib-2.0/gobject/gvaluearray.h:54): Use 'g_array_index' instead [-Werror=deprecated-declarations]
cc1: all warnings being treated as errors

make[3]: *** [libnm_util_la-nm-param-spec-specialized.lo] Error 1
make[3]: *** Waiting for unfinished jobs....
mv -f .deps/libnm_util_la-nm-setting-bond.Tpo .deps/libnm_util_la-nm-setting-bond.Plo
mv -f .deps/libnm_util_la-crypto.Tpo .deps/libnm_util_la-crypto.Plo
mv -f .deps/libnm_util_la-nm-setting-connection.Tpo .deps/libnm_util_la-nm-setting-connection.Plo
mv -f .deps/libnm_util_la-nm-setting.Tpo .deps/libnm_util_la-nm-setting.Plo
mv -f .deps/libnm_util_la-nm-connection.Tpo .deps/libnm_util_la-nm-connection.Plo
mv -f .deps/libnm_util_la-nm-setting-8021x.Tpo .deps/libnm_util_la-nm-setting-8021x.Plo
make[3]: Leaving directory `/home/wena/checkout/gnome/NetworkManager/libnm-util'
make[2]: Leaving directory `/home/wena/checkout/gnome/NetworkManager/libnm-util'
make[1]: Leaving directory `/home/wena/checkout/gnome/NetworkManager'
make[2]: *** [all-recursive] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2

My workaround:

$ echo 'module_autogenargs["NetworkManager"] = "--enable-more-warnings=no"' >> ~/.jhbuildrc