CMakeDetermineSystem: Clarify comment when CMAKE_SYSTEM_NAME is already set

Modules/CMakeDetermineSystem.cmake

@@ -177,11 +177,9 @@ if(CMAKE_TOOLCHAIN_FILE)
   endif()
 endif()
 
-
-# if CMAKE_SYSTEM_NAME is here already set, either it comes from a toolchain file
-# or it was set via -DCMAKE_SYSTEM_NAME=...
-# if that's the case, assume we are crosscompiling
 if(CMAKE_SYSTEM_NAME)
+  # CMAKE_SYSTEM_NAME was set by a toolchain file or on the command line.
+  # Assume it set CMAKE_SYSTEM_VERSION and CMAKE_SYSTEM_PROCESSOR too.
   if(NOT DEFINED CMAKE_CROSSCOMPILING)
     set(CMAKE_CROSSCOMPILING TRUE)
   endif()
@@ -193,6 +191,7 @@ elseif(CMAKE_VS_WINCE_VERSION)
   set(CMAKE_CROSSCOMPILING TRUE)
   set(PRESET_CMAKE_SYSTEM_NAME TRUE)
 else()
+  # Build for the host platform and architecture by default.
   set(CMAKE_SYSTEM_NAME      "${CMAKE_HOST_SYSTEM_NAME}")
   if(NOT DEFINED CMAKE_SYSTEM_VERSION)
     set(CMAKE_SYSTEM_VERSION "${CMAKE_HOST_SYSTEM_VERSION}")