Improve #include guard macros

Avoid using reserved identifiers such as `_DOHINT_H` (fixes #1989),
fix cases where the name doesn't match the filename, and make the naming
more consistent and less likely to collide with include guards in other
headers.

Source/DOH/doh.h

@@ -11,8 +11,8 @@
  *     This file describes of the externally visible functions in DOH.
  * ----------------------------------------------------------------------------- */
 
-#ifndef _DOH_H
-#define _DOH_H
+#ifndef SWIG_DOH_H
+#define SWIG_DOH_H
 
 #ifndef MACSWIG
 #include "swigconfig.h"
@@ -503,4 +503,4 @@ extern void DohMemoryDebug(void);
 # pragma GCC poison abort exit
 #endif
 
-#endif				/* DOH_H */
+#endif				/* SWIG_DOH_H */

Source/DOH/dohint.h

@@ -11,8 +11,8 @@
  *     This file describes internally managed objects.
  * ----------------------------------------------------------------------------- */
 
-#ifndef _DOHINT_H
-#define _DOHINT_H
+#ifndef SWIG_DOHINT_H
+#define SWIG_DOHINT_H
 
 #include "doh.h"
 
@@ -128,4 +128,4 @@ typedef struct {
 extern DOH *DohObjMalloc(DohObjInfo *type, void *data);	/* Allocate a DOH object */
 extern void DohObjFree(DOH *ptr);	/* Free a DOH object     */
 
-#endif				/* DOHINT_H */
+#endif				/* SWIG_DOHINT_H */