Cannot build with opencl-headers 2.x as it is not backwards compatible with 1.x.
2.0 ships in current debian (testing and jessie-backports) as the only opencl-headers package.
Haven't looked into it much further yet.
Gist to cl2.hpp
https://gist.github.com/lopho/aa80c477518c65d05b86
Gist to cl.hpp -> cl2.hpp diff
https://gist.github.com/lopho/569aa7d9c1029a9d70d9
https://www.khronos.org/registry/cl/api/2.0/
(for anybody else on debian testing, downgrade to opencl-headers_1.2-svn26009-1_all.deb from here:
http://ftp.debian.org/debian/pool/main/k/khronos-opencl-headers/)