benejson: new package

Added C/C++ JSON parsing package benejson.

[Thomas:
 - Get rid of the usage of BENEJSON_SCONS_OPTS, since this variable is
   never defined.
 - Remove BENEJSON_SCONS_ENV, and pass the options directly in the
   definition of <pkg>_BUILD_CMDS.
 - Fix indentation of BENEJSON_BUILD_CMDS.]

Signed-off-by: Dave Bender <bender@benegon.com>
Signed-off-by: David Bender <codehero@gmail.com>
[yann.morin.1998@free.fr: fix static/shared install; enhance help entry;
 fix extra space; add hash]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>