Caching for OpenCL builds should detect when a rebuild is necessary

[ancahamuraru]

The current caching implemented in ocl_compile_program(...) needs to be improved so that it will rebuild the code if:

• the OpenCL kernels change or
• the OpenCL compilation flags change.

[ancahamuraru]

See check_ocl_cache function.

[ancahamuraru]

See also #86