Like the ability to specify the vectorization level, users of CMake can
now specify what GPU architectures they want to build for. Most users
should just use the default 'native'.
* Support a REQUIRED flag that only gives an error if that flag is given.
* Move common configuration required for all devices (such as boost) to a
special device named Base.
* Make CUDA always capitalized to be consistent with the other CMake
variables.
* Rather than call include_directories, set a variable named
VTKm_INCLUDE_DIRS. This is consistent with how most CMake packages work.
* Make a CMake variable named VTKm_LIBRARIES containing all the
libraries the configured devices need.
* Automatically configure supported devices when loading the VTK-m
package in CMake.
