[dev branch] Cmake error

Both Windows and Linux builds fail with cmake. (Of course you can use make on Linux, I just tried cmake on Linux to verify the problem.)

> CMake Error at cmake/ProtoBuf.cmake:28 (message):
> Error: pslite_protobuf_generate_cpp_py() called without any proto files
> Call Stack (most recent call first):
> CMakeLists.txt:23 (pslite_protobuf_generate_cpp_py)

I printed out the parameters:

> proto_gen_folder=/home/hct/test/ps-lite/include prot_srcs= proto_hdrs=
> proto_python= project_src_dir=/home/hct/test/ps-lite proto_files=