-F/--force-define and relative %define/-D/--define priorities.

* NEWS (2.5): Add documentation to -D/--define entry.
* build-aux/cross-options.pl: Hard-code association of
--force-define with %define.
* doc/bison.texinfo (Decl Summary): In %define entry,
cross-reference command-line options.
(Bison Options): Add documentation to -D/--define entry.
(Option Cross Key): Widen column for --force-define row.
* src/getargs.c (usage): Document -F/--force-define.  Realign
options in output.
(short_options, long_options, getargs): Parse -F/--force-define,
and update muscle_percent_define_insert invocations.
* src/muscle-tab.h (muscle_percent_define_how): New enum type.
(muscle_percent_define_insert): Add argument with that type.
* src/muscle-tab.c (muscle_percent_define_insert): Implement
-F/--force-define behavior and priorities.
(muscle_percent_define_ensure): Update
muscle_percent_define_insert invocation.
* src/parse-gram.y (prologue_declaration): Update
muscle_percent_define_insert invocations.
* tests/input.at (`%define, --define'): Rename to...
(`%define, --define, --force-define'): ... this and extend.
(cherry picked from commit 34d4193815)

Conflicts:

	src/getargs.c
	src/parse-gram.c
	src/parse-gram.h
	src/parse-gram.y

src/muscle-tab.h

@@ -126,12 +126,25 @@ void muscle_boundary_grow (char const *key, boundary bound);
 void muscle_user_name_list_grow (char const *key, char const *user_name,
                                  location loc);
 
-/* Define the muscle for the %define variable VARIABLE appearing at
-   VARIABLE_LOC with value VALUE.  Warn if VARIABLE is already defined.
-   Record this as a user occurrence of VARIABLE by invoking
+/* Indicates whether a variable's value was specified with -D/--define, with
+   -F/--force-define, or in the grammar file.  */
+typedef enum {
+  MUSCLE_PERCENT_DEFINE_D = 0, MUSCLE_PERCENT_DEFINE_F,
+  MUSCLE_PERCENT_DEFINE_GRAMMAR_FILE
+} muscle_percent_define_how;
+
+/* Define the muscles for %define variable VARIABLE with VALUE specified
+   at VARIABLE_LOC in the manner HOW unless it was specified in the
+   grammar file while the previous definition for VARIABLE was specified
+   with -F/--force-define.  Warn if a previous definition is being
+   overridden and the new definition is specified in the grammar file.
+   (These rules support the documented behavior as long as command-line
+   definitions are processed before grammar file definitions.)  Record
+   this as a user occurrence of VARIABLE by invoking
    muscle_user_name_list_grow.  */
 void muscle_percent_define_insert (char const *variable, location variable_loc,
-                                   char const *value);
+                                   char const *value,
+                                   muscle_percent_define_how how);
 
 /* Make sure that VARIABLE is set to the boolean VALUE.  Warn on mismatches
    only, but accept repeated declaration.  Used for backward compatibility