See gdb ChangeLog entry with header:

Thu Jul 25 19:41:31 1996  Fred Fish  <fnf@cygnus.com>

for a rather huge set of changes.  I was going to put them here, but it
made cvs dump core.  :-(

gdb/m2-valprint.c

@@ -22,6 +22,7 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.  */
 #include "symtab.h"
 #include "gdbtypes.h"
 #include "valprint.h"
+#include "m2-lang.h"
 
 /* FIXME:  For now, just explicitly declare c_val_print and use it instead */