Use system http-parser if available

Changes to help packaging, part II: use the system's installation of http-parser if available. Compiles and tests pass with both http-parser 1.0 and 2.0.

Do you have any plans to submit the changes you've made to http-parser upstream and update the bundled version? Most distributions don't allow bundled libraries in packages so this can be/is an issue for packagers. What about xdiff?