Add global generator factory method to get list of known platforms

Add a `cmGlobalGeneratorFactory::GetKnownPlatforms` method to return
a list of known possible values for `CMAKE_GENERATOR_PLATFORM`.
Implement the method for each generator by referencing the list of
possible values documented in `Help/generator/*.rst` for it.

Co-Author: Julien Jomier <julien.jomier@kitware.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/Source/cmake.h b/Source/cmake.h
index b37cccd..d67f835 100644
--- a/Source/cmake.h
+++ b/Source/cmake.h
@@ -104,6 +104,7 @@ public:
     std::string extraName;
     bool supportsToolset;
     bool supportsPlatform;
+    std::vector<std::string> supportedPlatforms;
     bool isAlias;
   };