imv-swid: Use pkg-config to check for libjson-c

imv-swid: Use pkg-config to check for libjson-c

The package/library is called libjson-c on recent distributions.
Some like Ubuntu 14.04 provide symlinks with the old name but these
will eventually disappear.  Using pkg-config allows us to easily check
for it (with a fallback) and configure the proper compiler flags.

Fixes #663.