Add a define to Firestore C++ to work around a grpc Windows compile error. (#238)

* Add -D_WIN32_WINNT=0x0600 to Firestore compiler options, required on Windows.

Author: jonsimantov

firestore/CMakeLists.txt

@@ -279,10 +279,17 @@ else()
   )
 endif()
 
+set(FIREBASE_FIRESTORE_CPP_DEFINES -DINTERNAL_EXPERIMENTAL=1)
+
+if (WIN32 AND NOT ANDROID AND NOT IOS)
+  # On Windows, gRPC gives a compiler error in firebase_metadata_provider_desktop.cc
+  # unless _WIN32_WINNT is defined to this value (0x0600, Windows Vista).
+  set(FIREBASE_FIRESTORE_CPP_DEFINES ${FIREBASE_FIRESTORE_CPP_DEFINES} -D_WIN32_WINNT=0x0600)
+endif()
 
 target_compile_definitions(firebase_firestore
   PRIVATE
-    -DINTERNAL_EXPERIMENTAL=1
+    ${FIREBASE_FIRESTORE_CPP_DEFINES}
 )
 
 if(ANDROID)