The default max nesting depth of json-c is 32, which can cause some valid trees to fail to be parsed, so increase that. Also instead of segfaulting, just abort and print the error returned by json-c.
Increase _POSIX_SOURCE value where needed.
Increase _XOPEN_SOURCE value where needed.
Conditionally link to libcap (only on Linux).
Possibly some trailing whitespace fixes (automatic).