-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/parse-gram.y

@@ -229,7 +229,8 @@ prologue_declaration:
     }
 | "%define" variable content.opt
     {
-      muscle_percent_define_insert ($2, @2, $3);
+      muscle_percent_define_insert ($2, @2, $3,
+                                    MUSCLE_PERCENT_DEFINE_GRAMMAR_FILE);
     }
 | "%defines"                       { defines_flag = true; }
 | "%defines" STRING