Improve compiler compatibility (#743)

* fix returns

* use standard C headers

* struct/class mismatch

* explicit this in template

* switch variable scope

* C standard compliance

* & l-value

include/JSystem/JMessage/processor.h

@@ -7,7 +7,7 @@ namespace JMessage {
 
 struct TReference;
 struct TControl;
-struct TResource;
+class TResource;
 
 struct TProcessor {
 public:
@@ -80,7 +80,7 @@ public:
     static const char* process_select_(TProcessor*);
 
     void pushCurrent(const char*);
-    const char* popCurrent();
+    void popCurrent();
     void on_select_begin(OnSelectBeginCallBack, const void*, const char*, u32);
     void on_select_end();
     void on_select_separate();