Fix Jansson dependency importing

I realy wonder how that couldn't explode earlier.

First, imported targets don't propagate into upper dirs without GLOBAL, which was missing.
Second, set_propery cannot set multiple properties at once.
Third, one cannot link imported unknown libs into another library target.

This only worked because noone ever tried to build with a system supplied Jansson library on windows or OSX before.
On linux it just happens to work, because passing "jansson" to target_link_libraries makes it link against jansson.so, and the include files are in a default include dir.