docs: improvements to source code documentation (#1236)

src/version.h.in

@@ -1,10 +1,7 @@
-#ifndef INCLUDE_GUARD
-#define INCLUDE_GUARD
+#pragma once
 
 #define PROJECT_NAME "@PROJECT_NAME@"
 #define PROJECT_VER  "@PROJECT_VERSION@"
 #define PROJECT_VER_MAJOR "@PROJECT_VERSION_MAJOR@"
 #define PROJECT_VER_MINOR "@PROJECT_VERSION_MINOR@"
 #define PROJECT_VER_PATCH "@PROJECT_VERSION_PATCH@"
-
-#endif // INCLUDE_GUARD