(start_of_data): If DATA_START is defined, prefer

its value over other approaches.

src/mem-limits.h

@@ -79,12 +79,18 @@ extern POINTER start_of_data ();
 #define EXCEEDS_LISP_PTR(ptr) ((EMACS_UINT) (ptr) >> VALBITS)
 #endif
 
+#ifdef DATA_START
+#define start_of_data() ((char *)DATA_START)
+#endif
+
 #ifdef BSD_SYSTEM
 #ifndef DATA_SEG_BITS
+#ifndef DATA_START
 extern char etext;
 #define start_of_data() &etext
 #endif
 #endif
+#endif
 
 #else  /* not emacs */
 extern char etext;