build: use new name for libjson-c

libjson-c changed its install name a while back to avoid clashes with other
implementations: https://github.com/json-c/json-c/commit/30dd367c

We can probably just change our pkg_deps now to use the new name, since the
new version is available for Debian stable via backports, and in the most
recent Ubuntu LTS.

https://gna.org/bugs/?20817

(Thanks, akbjker)
temporary #24 (provided by @Pivetta)
remove no longer needed workaround introduced in 7e648bc8807fe232c7182fb092a3c7265e792fbf
Merge branch 'master' of ../libmypaint