Add CMake build system for better cross-platform support

The C bindings primary build system is Autotools. This obviously does not work for Windows, and so the original port to Windows simply added a Visual Studio solution to the project, splitting the build system. As new versions of Visual Studio have come along, new (probably auto-converted) solutions have come along (see zookeeper.sln vs zookeeper-vs2013.sln). When Mesos started being ported to Windows, a Visual Studio 2015 solution was needed, and the previous developer created yet another solution, and setup Mesos' build to patch ZooKeeper and add the 2015 solution. Now Visual Studio 2017 was released, and in the process of moving Mesos ahead, I realized that I would either have to make yet another converted solution for ZooKeeper. So instead I tackled the root problem, and ported the Autotools build to CMake, which is a meta-build system which generates files for the in-use platform (whether it be Linux or Solaris or MacOS or Windows).

NOTE: I already have this patch, and will submit it. It has a couple TODOs, and some other things in it that were necessary for Mesos that may need to be pulled into separate patches.