Check for unrecognized %define variables similar to checking for

unrecognized %code qualifiers.  Check for redefined %define variables.
* data/bison.m4 (b4_check_for_unrecognized_names): New macro that
generalizes...
(b4_check_percent_code_qualifiers): ... this, which now wraps it.
(b4_check_percent_define_variables): New, also wraps it.
* data/glr.c: Unless glr.cc is wrapping glr.c, declare no valid %define
variables using b4_check_percent_define_variables.
* data/glr.cc, data/lalr1.cc: Declare the valid %define variables as
all those exercised in the test suite and all those listed in the
`Default values' section of c++.m4.  Are there others?
* data/push.c, data/yacc.c: Declare no valid %define variables.
* src/muscle_tab.c, src/muscle_tab.h (muscle_find_const): New function,
similar to muscle_find, but it works even when the muscle stores a
const value.
(muscle_grow_used_name_list): New function for constructing the used
name list muscles that b4_check_for_unrecognized_names requires.
* src/parse-gram.y (prologue_declaration): Warn if a variable is
%define'd more than once.  Define the b4_used_percent_define_variables
muscle with muscle_grow_used_name_list.
(grammar_declaration): Abbreviate %code code with
muscle_grow_used_name_list.
* tests/input.at (%define errors): New.

data/push.c

@@ -33,7 +33,12 @@ b4_use_push_for_pull_if([
   ])])
 
 m4_include(b4_pkgdatadir/[c.m4])
-b4_check_percent_code_qualifiers([[requires]], [[provides]], [[top]])
+
+b4_check_percent_define_variables
+
+b4_check_percent_code_qualifiers([[requires]],
+                                 [[provides]],
+                                 [[top]])
 
 ## ---------------- ##
 ## Default values.  ##