VS: Add VS_SOURCE_SETTINGS_<tool> target property

VS_SOURCE_SETTINGS_<tool> is a list of key value pairs that get written as item
metadata for any file associated with <tool> for that target. The entire value
of VS_SOURCE_SETTINGS_<tool> is evaluated as a generator expression. VS_SETTINGS
can override settings specified in this property.

Source/cmVisualStudio10TargetGenerator.cxx

@@ -1967,6 +1967,22 @@ void cmVisualStudio10TargetGenerator::WriteExtraSource(Elem& e1,
     }
   }
 
+  if (ParsedToolTargetSettings.find(tool) == ParsedToolTargetSettings.end()) {
+    const char* toolTargetProperty =
+      this->GeneratorTarget->Target->GetProperty("VS_SOURCE_SETTINGS_" +
+                                                 std::string(tool));
+    ConfigToSettings toolTargetSettings;
+    ParseSettingsProperty(toolTargetProperty, toolTargetSettings);
+
+    ParsedToolTargetSettings[tool] = toolTargetSettings;
+  }
+
+  for (const auto& configToSetting : ParsedToolTargetSettings[tool]) {
+    for (const auto& setting : configToSetting.second) {
+      toolSettings[configToSetting.first][setting.first] = setting.second;
+    }
+  }
+
   ParseSettingsProperty(sf->GetProperty("VS_SETTINGS"), toolSettings);
 
   if (!toolSettings.empty()) {

Source/cmVisualStudio10TargetGenerator.h

@@ -241,6 +241,7 @@ private:
   using ConfigToSettings =
     std::unordered_map<std::string,
                        std::unordered_map<std::string, std::string>>;
+  std::unordered_map<std::string, ConfigToSettings> ParsedToolTargetSettings;
   bool PropertyIsSameInAllConfigs(const ConfigToSettings& toolSettings,
                                   const std::string& propName);
   void ParseSettingsProperty(const char* settingsPropertyValue,