updates README build instructions

[bashbaug]

The current README build instructions are more complicated than they need to be. Much simpler build instructions can be used in most cases. So, document the simpler build instructions, instead.

Additionally:

• Document the OpenCL SDK CMake variables that may be used to control the build.
• Disable samples that require Vulkan by default, to reduce build dependencies. Note that OpenGL samples were already disabled by default.
• Add CI configs to test the simple README build instructions on Windows, Linux, and Mac.

see: #70