Currently the minimum required version of libnghttp2 is set to 1.4. This is wrong, it is supposed to be 1.40.0. But we want to to get it further down to 1.33.0 cause this version is widely available.
Also, make sure the http2_adaptor.c really does not compile if the available library version if less than 1.33.0. Instead http2_adaptor_none.c should compile. Right now, http2_adaptor.c complies if any version of libnghttp2 is found.