set c++ version

spec/datatypes.md

@@ -63,6 +63,9 @@ struct LibraryConfig {
 
     /// Include directories
     Array<string> include_dirs;
+
+    /// Optional list of properties
+    Array<Property> properties;
 };
 ```
 
@@ -97,6 +100,9 @@ struct ExecutableConfig {
 
     /// Include directories
     Array<string> include_dirs;
+
+    /// Optional list of properties
+    Array<Property> properties;
 };
 ```
 
@@ -176,4 +182,15 @@ struct TaskConfig {
     /// Shell script to be executed when this task is called
     string shell_script;
 };
+```
+
+## Property
+```c++
+struct Property {
+    /// Property name
+    std::string name;
+
+    /// Property value
+    std::string value;
+};
 ```