clover: Dynamically calculate __OPENCL_VERSION__ and CLC language version

clover: Dynamically calculate __OPENCL_VERSION__ and CLC language version

Use get_language_version to calculate default cl standard based on
device capabilities and -cl-std specified in build options.

v5; move dev_clc_version declaration from an earlier patch
v4: Squash the __OPENCL_VERSION__ and CLC language version patches
v3: (Jan) Allow device_version up to 2.2 while device_clc_version
    only goes to 2.0
    Use get_cl_version to calculate version instead
v2: Split out from the previous patch (Pierre)

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Pierre Moreau <pierre.morrow@free.fr>
CC: Jan Vesely <jan.vesely@rutgers.edu>