-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/parse-gram.y

@@ -227,7 +227,8 @@ prologue_declaration:
 | "%debug"                         { debug_flag = true; }
 | "%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
@@ -270,12 +271,14 @@ prologue_declaration:
          `%define api.pure' in a backward-compatible manner here.  First, don't
          complain if %pure-parser is specified multiple times.  */
       if (!muscle_find_const ("percent_define(api.pure)"))
-        muscle_percent_define_insert ("api.pure", @1, "");
+        muscle_percent_define_insert ("api.pure", @1, "",
+                                      MUSCLE_PERCENT_DEFINE_GRAMMAR_FILE);
       /* In all cases, use api.pure now so that the backend doesn't complain if
          the skeleton ignores api.pure, but do warn now if there's a previous
          conflicting definition from an actual %define.  */
       if (!muscle_percent_define_flag_if ("api.pure"))
-        muscle_percent_define_insert ("api.pure", @1, "");
+        muscle_percent_define_insert ("api.pure", @1, "",
+                                      MUSCLE_PERCENT_DEFINE_GRAMMAR_FILE);
     }
 | "%require" STRING             { version_check (&@2, $2); }
 | "%skeleton" STRING