Mongoose
https://github.com/cesanta/mongoose
event-driven non-blocking  Embedded Web Server
Very easy to integrate: 
just copy mongoose.c and mongoose.h files to your build tree
GitHub's most popular embedded web server and multi-protocol networking library
https://cesanta.com/docs/#minimal-http-server
https://cesanta.com/docs/#minimal-tcp-echo-server
https://github.com/cesanta/mongoose/blob/master/examples/http-restful-server/main.c


https://github.com/Qihoo360/evpp
A modern C++ network library for developing high performance network services in TCP/UDP/HTTP protocols.


https://github.com/atp798/EvHttp
cpp http server, wrapped of libevent
C++ wrapped http server based on libevent
https://www.straka.cn/blog/cpp-wrapped-http-server-based-on-libevent/


cpp-httplib
https://github.com/yhirose/cpp-httplib
A C++11 single-file header-only cross platform HTTP/HTTPS library.
It's extremely easy to setup. Just include the httplib.h file in your code!
NOTE: This is a multi-threaded 'blocking' HTTP library
https://gitee.com/dyyx/hellocode/tree/master/web/tech/cpp/demo/httplib