c, c++: provide a default definition for N_

In C/C++, N_ is a no-op.  Define it if the user didn't.
Suggested by Frank Heckenbach.
https://lists.gnu.org/r/bug-bison/2020-04/msg00010.html

* src/output.c (prepare_symbol_names): Rename has_translations as
has_translations_flag.
* data/skeletons/bison.m4 (b4_has_translations_if): New.
* data/skeletons/java.m4 (b4_trans): Use it.

* data/skeletons/glr.c, data/skeletons/lalr1.cc, data/skeletons/yacc.c
(N_): Provide a default definition.

data/skeletons/bison.m4

@@ -369,6 +369,7 @@ m4_define([b4_$3_if],
 # Expand IF-TRUE, if FLAG is true, IF-FALSE otherwise.
 b4_define_flag_if([defines])            # Whether headers are requested.
 b4_define_flag_if([glr])                # Whether a GLR parser is requested.
+b4_define_flag_if([has_translations])   # Whether some tokens are internalionalized.
 b4_define_flag_if([nondeterministic])   # Whether conflicts should be handled.
 b4_define_flag_if([token_table])        # Whether yytoken_table is demanded.
 b4_define_flag_if([yacc])               # Whether POSIX Yacc is emulated.

data/skeletons/glr.c

@@ -261,6 +261,11 @@ static YYLTYPE yyloc_default][]b4_yyloc_default;])[
 #  define YY_(Msgid) Msgid
 # endif
 #endif
+]b4_has_translations_if([
+#ifndef N_
+# define N_(Msgid) Msgid
+#endif
+])[
 
 #ifndef YYFREE
 # define YYFREE free

data/skeletons/java.m4

@@ -344,7 +344,7 @@ m4_define([b4_symbol_translate],
 # ----------------
 # Translate a string if i18n is enabled.  Avoid collision with b4_translate.
 m4_define([b4_trans],
-[m4_if(b4_has_translations, 0, [$1], [i18n($1)])])
+[b4_has_translations_if([i18n($1)], [$1])])
 
 
 

data/skeletons/lalr1.cc

@@ -523,6 +523,11 @@ m4_if(b4_prefix, [yy], [],
 #  define YY_(msgid) msgid
 # endif
 #endif
+]b4_has_translations_if([
+#ifndef N_
+# define N_(Msgid) Msgid
+#endif
+])[
 
 // Whether we are compiled with exception support.
 #ifndef YY_EXCEPTIONS

data/skeletons/yacc.c

@@ -463,6 +463,11 @@ typedef int yy_state_fast_t;
 #  define YY_(Msgid) Msgid
 # endif
 #endif
+]b4_has_translations_if([
+#ifndef N_
+# define N_(Msgid) Msgid
+#endif
+])[
 
 ]b4_attribute_define[