maybe add some include guards

examples/libc99/libc99.h

@@ -4,6 +4,9 @@
 // * c-string
 // * struct
 // Those are all considered POD-types
+#ifndef LIBPEPDATATYPES_LIBC99_H
+#define LIBPEPDATATYPES_LIBC99_H
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -34,4 +37,6 @@ extern "C" {
     void free_test_struct(Test_struct1* c_struct);
 #ifdef __cplusplus
 }
-#endif
+#endif
+
+#endif