Have you actually tried compiling it on Linux? I've been building Dolphin myself for the past year, and so far it's always compiled the OpenCL texture decoder just fine (though it's not particularly useful in its current state). Which part of the CMakeList.txt are you looking at (it's the one in the root directory of the git repo, right?)
If you look carefully at Line 480, it appears CMake will try to find existing libraries for OpenCL (which I assume includes the headers) on OS X. For everything else, looks like Dolphin just pulls the files from Externals. It should still be possible to compile the OpenCL texture decoder regardless of OS. I'm not very experienced in reading CMake scripts, so hopefully a dev will expand and correct what I've said.
If you look carefully at Line 480, it appears CMake will try to find existing libraries for OpenCL (which I assume includes the headers) on OS X. For everything else, looks like Dolphin just pulls the files from Externals. It should still be possible to compile the OpenCL texture decoder regardless of OS. I'm not very experienced in reading CMake scripts, so hopefully a dev will expand and correct what I've said.