boost::compute::system::default_device() supports the following
environment variables:
BOOST_COMPUTE_DEFAULT_DEVICE for device name
BOOST_COMPUTE_DEFAULT_PLATFORM for OpenCL platform name
BOOST_COMPUTE_DEFAULT_VENDOR for device vendor name
If one or more of these variables is set, then device that satisfies
all conditions gets selected. If such a device is unavailable, then
the first available GPU is selected. If there are no GPUs in the
system, then the first available CPU is selected. Otherwise,
default_device() returns null device.
The hello_world example is modified to use default_device() instead
of default_gpu_device().
