Includes ~2 years worth of fixes including security and build.
Note: this pulls directly from master, not a release branch/tag. See
https://github.com/Tencent/rapidjson/issues/1265
(cherry picked from commit 8a6c081df71cde3f1c55d5b7e5ef7d5d34b475dc)
Though we only need miniupnpc, rebasing and maintaining a miniupnpc-only
repo is unrealistic.
(cherry picked from commit 3c40eb768c292a4dec79d7dffd6685fd37940a2a)