cmake: Document CMAKE_SYSTEM_-variables, and add CMAKE_SYSTEM_VERSION

Aid user in understanding what CMake expects of:

 * Document and add CMAKE_SYSTEM_VERSION
 * Document CMAKE_SYSTEM_NAME
 * Document CMAKE_SYSTEM_PROCESSOR
 * Document BUILD_SHARED_LIBS
 * Document Policies CMP0002, CMP0079

CMAKE_SYSTEM_VERSION is required officially by CMake, but appears
unused -- at least now we are compliant.

Signed-off-by: Mark Ruvald Pedersen <mped@oticon.com>
2 files changed