Automatically Select `-cl-std` for Compiler Defines Test (#921)

* Automatically select the `-cl-std` build option (stop assuming
`CL2.0`) in the `compiler_defines_for_extensions` test. This will allow
us to test `test_compiler_defines_for_extensions` with an OpenCL-3.0
driver that optionally might not support OpenCL C 2.0. Use the CTS
framework to automatically select the latest version of OpenCL C
supported by the device.