Fix error when using provided jansson

[orgads]

Unknown CMake command "PKG_SEARCH_MODULE".

[orgads]

@benmcollins What is the right way to pass the path? With 1.13, I passed -Djansson_DIR="$JANSSON_CMAKE" and it worked. How can it be done with pkgconfig?

[benmcollins]

You may need to tell CMAKE where your pkgconfig lib dir is. I've had to do this with auto tools, but I'm not sure how to do it with CMAKE. I don't really maintain the build method.