Normalize system directories from the interface target property

The input dir being tested is normalized, so ensure that the entries
in the vector are normalized too (eg no trailing slash).

Source/cmGeneratorTarget.cxx

@@ -80,6 +80,12 @@ bool cmGeneratorTarget::IsSystemIncludeDirectory(const char *dir,
                                         config, false, this->Target,
                                         &dagChecker), result);
       }
+    for(std::vector<std::string>::iterator li = result.begin();
+        li != result.end(); ++li)
+      {
+      cmSystemTools::ConvertToUnixSlashes(*li);
+      }
+
     IncludeCacheType::value_type entry(config_upper, result);
     iter = this->SystemIncludesCache.insert(entry).first;
     }