Adds support for selecting devices with environment variables

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().