bullet3fluids: cmake build && fix gcc compilation errors

Hi rtrius,

This changeset does pretty much what it says. Now the code compiles cleanly with cmake (at least in Ubuntu12.04).

Of note is that I added a FindOpenCL.cmake and corresponding cmake library entries. I've noticed that the lua system uses some special settings regarding the OpenCL library. CMake allows for manual selection of the include dir and the actual library, so I thought it best to stick with the regular cmake detection mechanism.

all the best,
João