Ninja: Refactor detection of MinGW tools on Windows

Check for CMAKE_COMPILER_IS_MINGW only after enabling a language when it
might actually be set.  Previously this worked by accident because the
check for working compiler or a second language enabled would cause the
code path to be taken.

Store UsingMinGW as an instance member of cmGlobalNinjaGenerator so that
it is reset on each reconfigure.  Otherwise cmake-gui cannot switch
between build trees for MinGW or non-MinGW tools.

Source/cmNinjaTargetGenerator.cxx

@@ -175,7 +175,7 @@ cmNinjaTargetGenerator::ComputeFlagsForObject(cmSourceFile const* source,
                                         // needed by cmcldeps
                                             false,
                                             this->GetConfigName());
-    if(cmGlobalNinjaGenerator::IsMinGW())
+    if (this->GetGlobalGenerator()->IsMinGW())
       cmSystemTools::ReplaceString(includeFlags, "\\", "/");
 
     this->LocalGenerator->AppendFlags(languageFlags, includeFlags);