clc: bring back opencl-c.h and stop requiring clc for clover or crocus

this MR contains three fixes for different issues:

1. clc actually needs to include opencl-c.h with certain extensions as otherwise OpenCL C code fails to compile. This was worked around with some of the OpenCL C tooling anv is using, but it's actually up to clc to decide when it's needed.
2. Enabling clover pulled in clc since recently, rework the build system to not do that.
3. Building hasvk or crocus pulled in src/intel/shaders/meson.build but failed to build due to missing prog_intel_clc.