-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

NEWS

@@ -59,17 +59,22 @@ Bison News
 
 ** %define can now be invoked via the command line.
 
-  Each of these bison command-line options
+  Each of these command-line options
 
-    -D NAME=VALUE
-    --define=NAME=VALUE
+    -D NAME[=VALUE]
+    --define=NAME[=VALUE]
+
+    -F NAME[=VALUE]
+    --force-define=NAME[=VALUE]
 
   is equivalent to this grammar file declaration
 
-    %define NAME "VALUE"
+    %define NAME ["VALUE"]
 
-  for any NAME and VALUE.  Omitting `=VALUE' on the command line is
-  equivalent to omitting `"VALUE"' in the declaration.
+  except that the manner in which Bison processes multiple definitions
+  for the same NAME differs.  Most importantly, -F and --force-define
+  quietly override %define, but -D and --define do not.  For further
+  details, see the section "Bison Options" in the Bison manual.
 
 ** %define variables renamed.