Our bundled http-parser includes bugfixes, therefore we should prefer
our http-parser until such time as we can identify that the system
http-parser has these bugfixes (using a version check).

Since these bugs are - at present - minor, retain the ability for users
to force that they want to use the system http-parser anyway.  This does
change the cmake specification so that people _must_ opt-in to the new
behavior knowingly.