Building on a Mac OS X (Intel), the inclusion of raspell.h in each of aspell.c, dictinfo.c, and raspell.c causes cAspell, cAspellError, and cDictInfo to be define multiple times.
I made these changes to raspell.c and raspell.h:
rab:ext $ diff -udb raspell.c{~,}
--- raspell.c~ 2005-08-20 14:51:18.000000000 -0400
+++ raspell.c 2006-11-16 10:15:52.000000000 -0500
@@ -1,4 +1,5 @@
+#define IN_RASPELL_C
#include "raspell.h"
extern void Init_dictinfo();
rab:ext $ diff -udb raspell.h{~,}
--- raspell.h~ 2005-08-20 14:51:18.000000000 -0400
+++ raspell.h 2006-11-16 10:09:25.000000000 -0500
@@ -4,10 +4,16 @@
#include <ruby.h>
#include <aspell.h>
-VALUE cAspell;
-VALUE cDictInfo;
+#ifdef IN_RASPELL_C
+#define MAYBE /**/
+#else
+#define MAYBE extern
+#endif
-VALUE cAspellError;
+MAYBE VALUE cAspell;
+MAYBE VALUE cDictInfo;
+
+MAYBE VALUE cAspellError;
#endif
