This set of patches fixes builds from master

This solves two issues:

1) sizeof(fields) in zmap:52 could not be calculated, it isn't clear how this was building for anyone else given pedantic mode
2) the preferred way to build against json-c is using pkg-config[1], but since that was factored out of the previous patch, a smaller change was made to specify the default include directory (-I/usr/include/json-c/) and link to the right library name (json-c vs json).