package/poke: add optional json-c dependency

From [1]:
"The poke machine-interface uses a protocol to communicate with
clients, which is based on JSON.  poke uses the json-c library to
parse and build JSON strings.
If this library is not found, poke will build without machine-interface
support."

[1] https://git.savannah.nongnu.org/cgit/poke.git/tree/DEPENDENCIES?h=releases/poke-1.2#n55



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Jose E. Marchesi <jemarch@gnu.org>
Cc: Luca Saiu <positron@gnu.org>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>