Ninja: use slashes for include dirs, so also slahes are in the .d files

Source/cmGlobalNinjaGenerator.h

@@ -147,6 +147,9 @@ public:
                            const cmNinjaDeps& targets,
                            const std::string& comment = "");
 
+
+  static bool IsMinGW() { return UsingMinGW; }
+
 public:
   /// Default constructor.
   cmGlobalNinjaGenerator();

Source/cmNinjaTargetGenerator.cxx

@@ -154,6 +154,8 @@ cmNinjaTargetGenerator::ComputeFlagsForObject(cmSourceFile *source,
                                               language.c_str());
   std::string includeFlags =
     this->LocalGenerator->GetIncludeFlags(includes, language.c_str(), false);
+  if(cmGlobalNinjaGenerator::IsMinGW())
+    cmSystemTools::ReplaceString(includeFlags, "\\", "/");
   this->LocalGenerator->AppendFlags(flags, includeFlags.c_str());
   }