added AMD OpenCL include files

When I tried to build Wings, I couldn't build cl, because it couldn't find the AMD OpenCL header files on my Ubuntu 12.10 box.

Because I didn't want to mess up too much with my base system (add links for example), I just modified the `c_src/Makefile` by adding the "official AMD OpenGL include directory" `/opt/AMDAPP/include` to the `CFLAGS` used on Linux.