yacc.c: no longer support YYERROR_VERBOSE

Supporting YYERROR_VERBOSE via cpp is a nuisance: m4 is in charge of
handling alternatives.  When adding more options for %define
parse.error, supporting both CPP and M4 is too complex.  Anyway,
YYERROR_VERBOSE was deprecated long ago.

* data/skeletons/yacc.c: Use m4 only to handle verbose/simple error
messages.

tests/conflicts.at

@@ -309,7 +309,6 @@ AT_DATA_GRAMMAR([input.y],
 #include <string.h>
 #include <assert.h>
 
-#define YYERROR_VERBOSE 1
 ]AT_YYERROR_DEFINE[
 /* The current argument. */
 static const char *input;
@@ -324,6 +323,8 @@ yylex (void)
 
 %}
 
+%define parse.error verbose
+
 %nonassoc '<' '>'
 
 %%