Update and modernize some aspects of CMake (#92)

Update and modernize some aspects of CMake (#92)

* Update and modernize some aspects of CMake

Ease integration with projects targeting more recent CMake versions:

* Update minimum required CMake version to 3.1.
* Specify a version `1.2` to the `project` CMake command, this silences
  developer warnings when the OpenCL-ICD-Loader is integrated in another
  project using `add_subdirectory`.
* Specify include directory on the `OpenCL` target, this removes the
  need to set the OpenCL-Headers include directory globally (old CMake)
  or set them manually per target which links the `OpenCL` target
  library.

* Update AppVeyor project config