Support for OCL_ICD_ASSUME_ICD_EXTENSION

[jrprice]

ocl-icd provides an OCL_ICD_ASSUME_ICD_EXTENSION environment variable which tells it to skip the check for cl_khr_icd when loading ICDs. There are some users that are depending on this functionality (e.g. to target implementations that don't have a fully compliant ICD implementation) that are unable to use the Khronos ICD Loader.

This would be pretty straightforward to implement and I think is low risk (off by default, requires explicit opt-in). Would there be any objections adding this functionality here?