-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 invocation.
* 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.
* 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.

src/muscle_tab.h

@@ -123,12 +123,25 @@ 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);
 
 /* Mimic b4_percent_define_get in ../data/bison.m4 exactly.  That is, if the
    %define variable VARIABLE is defined, return its value.  Otherwise, return