clover/meson: Define OpenCL header macros

Rather than defining the macros any time right before including an
OpenCL header, set Meson to define them for the whole clover project.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Acked-by: Francisco Jerez <currojerez@riseup.net>
Tested-by: Marge Bot <mesa/mesa!3137>
Part-of: <mesa/mesa!3137>
38 jobs for !3137 with fix_#2443 in 19 minutes and 2 seconds (queued for 3 seconds)
merge request