make unit/regression run OpenCL tests for each given context
Auto-detect available OpenCL contexts and run unit/regression tests for all of them
Don't use recursive make variables with $(shell)
Merge pull request #129 from OP2/run-opencl-tests-for-each-context

make unit/regression run OpenCL tests for each given context.

By default contexts are detected automatically but can be overridden by
setting or exporting OPENCL_CTXS to a space-separated list of contexts.