Help: Document the WINCE variable

Help/manual/cmake-variables.7.rst

@@ -192,6 +192,7 @@ Variables that Describe the System
    /variable/MSVC_VERSION
    /variable/UNIX
    /variable/WIN32
+   /variable/WINCE
    /variable/WINDOWS_PHONE
    /variable/WINDOWS_STORE
    /variable/XCODE_VERSION

Help/variable/WINCE.rst

@@ -0,0 +1,5 @@
+WINCE
+-----
+
+True when the :variable:`CMAKE_SYSTEM_NAME` variable is set
+to ``WindowsCE``.