muscle: enforce definition syntax for keyword variables

* src/muscle-tab.c (muscle_percent_define_get_kind)
(muscle_percent_define_check_kind): New.
(muscle_percent_define_default): Variables with a default value are
of "keyword" kind.
(muscle_percent_define_flag_if, muscle_percent_define_check_values):
Check that the variable is of keyword kind.
* data/bison.m4: Likewise, but in M4.  That is to say...
(b4_percent_define_default): Define the kind when the variable is undefined.
(b4_percent_define_check_kind): Use a better error message.
(_b4_percent_define_check_values, _b4_percent_define_check_values):
Former "enum" variables should be defined using the keyword syntax.
* doc/bison.texi: Update.
A couple of fixes.
* tests/input.at (%define keyword variables): New.

tests/input.at

@@ -1463,6 +1463,39 @@ m4_popdef([AT_TEST])
 AT_CLEANUP
 
 
+## --------------------------- ##
+## %define keyword variables.  ##
+## --------------------------- ##
+
+AT_SETUP([["%define" keyword variables]])
+
+m4_pushdef([AT_TEST],
+[AT_DATA([input.y],
+[[%define api.pure                ]$1[true]$2[
+%define api.push-pull             ]$1[both]$2[
+%define lr.default-reduction      ]$1[most]$2[
+%define lr.keep-unreachable-state ]$1[true]$2[
+%define lr.type                   ]$1[lalr]$2[
+%%
+exp: %empty
+]])
+
+AT_BISON_CHECK([[input.y]], [0], [],
+[[input.y:5.9-15: warning: %define variable 'lr.type' requires keyword values [-Wdeprecated]
+input.y:3.9-28: warning: %define variable 'lr.default-reduction' requires keyword values [-Wdeprecated]
+input.y:4.9-33: warning: %define variable 'lr.keep-unreachable-state' requires keyword values [-Wdeprecated]
+input.y:2.9-21: warning: %define variable 'api.push-pull' requires keyword values [-Wdeprecated]
+input.y:1.9-16: warning: %define variable 'api.pure' requires keyword values [-Wdeprecated]
+]])
+])
+
+AT_TEST(["], ["])
+AT_TEST([{], [}])
+m4_popdef([AT_TEST])
+
+AT_CLEANUP
+
+
 ## ------------------------ ##
 ## %define enum variables.  ##
 ## ------------------------ ##