hhvm: Detect json-c parser

Add detection for a json-c powered json_decode() implementation. Default
loose parsing behavior was previously attributed to HHVM, but additional
investigation has found that this is actually a "feature" of json-c.
HHVM optionally links with json-c for distributions that wish to avoid
being encumbered by the "shall be used for Good, not Evil" clause of the
JSON.org parser's license. The test case implementation stays the same,
but a feature flag style check is added to determine if the parser being
used is lenient or not.

Bug: 72778
Change-Id: Ieb8dddd2e324c90f45f2fc27ac13ab5fff733302
Merge "hhvm: Detect json-c parser"