Allow CMAKE_SYSTEM_VERSION to be set without CMAKE_SYSTEM_NAME

Teach CMakeDetermineSystem to check for a CMAKE_SYSTEM_VERSION setting
even when CMAKE_SYSTEM_NAME is not set.  This will allow builds on the
host OS to target other versions of the OS without full cross-compiling.

Help/variable/CMAKE_SYSTEM_VERSION.rst

@@ -12,6 +12,13 @@ then ``CMAKE_SYSTEM_VERSION`` is by default set to the same value as the
 :variable:`CMAKE_HOST_SYSTEM_VERSION` variable so that the build targets
 the host system version.
 
+In the case of a host build then ``CMAKE_SYSTEM_VERSION`` may be set
+explicitly when first configuring a new build tree in order to enable
+targeting the build for a different version of the host operating system
+than is actually running on the host.  This is allowed and not considered
+cross compiling so long as the binaries built for the specified OS version
+can still run on the host.
+
 System Version for Cross Compiling
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

Modules/CMakeDetermineSystem.cmake

@@ -123,7 +123,9 @@ elseif(CMAKE_VS_WINCE_VERSION)
   set(PRESET_CMAKE_SYSTEM_NAME TRUE)
 else()
   set(CMAKE_SYSTEM_NAME      "${CMAKE_HOST_SYSTEM_NAME}")
-  set(CMAKE_SYSTEM_VERSION   "${CMAKE_HOST_SYSTEM_VERSION}")
+  if(NOT DEFINED CMAKE_SYSTEM_VERSION)
+    set(CMAKE_SYSTEM_VERSION "${CMAKE_HOST_SYSTEM_VERSION}")
+  endif()
   set(CMAKE_SYSTEM_PROCESSOR "${CMAKE_HOST_SYSTEM_PROCESSOR}")
   set(CMAKE_CROSSCOMPILING FALSE)
   set(PRESET_CMAKE_SYSTEM_NAME FALSE)