CMake: Report whether generators support platforms

Source/cmGlobalGeneratorFactory.h

Source/cmGlobalGhsMultiGenerator.h

Source/cmGlobalNinjaGenerator.h

Source/cmGlobalUnixMakefileGenerator3.h

Source/cmGlobalVisualStudio10Generator.cxx

Source/cmGlobalVisualStudio11Generator.cxx

Source/cmGlobalVisualStudio12Generator.cxx

Source/cmGlobalVisualStudio14Generator.cxx

Source/cmGlobalVisualStudio7Generator.h

Source/cmGlobalVisualStudio8Generator.cxx

Source/cmGlobalVisualStudio9Generator.cxx

Source/cmGlobalXCodeGenerator.cxx

Source/cmake.cxx

Source/cmake.h