Help: add missing MINGW variable documentation

Help/manual/cmake-variables.7.rst

@@ -181,6 +181,7 @@ Variables that Describe the System
    /variable/CMAKE_SYSTEM_VERSION
    /variable/CYGWIN
    /variable/ENV
+   /variable/MINGW
    /variable/MSVC10
    /variable/MSVC11
    /variable/MSVC12

Help/variable/MINGW.rst

@@ -0,0 +1,6 @@
+MINGW
+-----
+
+True when using MinGW
+
+Set to true when the compiler is some version of MinGW.