Currently there are multiple features that require Jansson support,
but WITH_JANSSON=1 is set only when configuring with
--enable-json-validation.  Therefore a build with
"--disable-json-validation --enable-ovs" fails.

The availability of Jansson (WITH_JANSSON) should only be used:

 - to check if dependent features can be enabled
 - to determine compiler and linker flags in the Makefile
 - in nm-jansson.h to define compatibility functions if needed

Everything else must be controlled by a configure switch.

https://bugzilla.gnome.org/show_bug.cgi?id=790233