diagnostics: clean up convention for colored diagnostics

* data/diagnostics.css: Rename as...
* data/bison-default.css: this.
Add the GPL header.
This is the convention followed by Bruno Haible in gettext.
Adjust dependencies.
* src/complain.c (complain_init_color): Use BISON_STYLE instead of
BISON_DIAGNOSTICS_STYLE.

NEWS

@@ -15,10 +15,10 @@ GNU Bison NEWS
 
 *** Colored diagnostics
 
-  As an experimental feature, Bison now generates colored diagnostics,
-  controlled by the new options --color and --style.
+  As an experimental feature, diagnostics are now colored, controlled by the
+  new options --color and --style.
 
-  Install the libtextstyle library before configuring Bison to use them.
+  To use them, install the libtextstyle library before configuring Bison.
   It is available from
 
     https://alpha.gnu.org/gnu/gettext/
@@ -34,12 +34,13 @@ GNU Bison NEWS
 
   To customize the styles, create a CSS file similar to
 
-    /* black-and-white.css */
+    /* bison-bw.css */
     .warning   { }
     .error     { font-weight: 800; text-decoration: underline; }
     .note      { }
 
-  then invoke bison with --style=black-and-white.css.
+  then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
+  environment variable to "bison-bw.css".
 
 *** Disabling output
 
@@ -3623,7 +3624,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
  LocalWords:  redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
  LocalWords:  pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
  LocalWords:  Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
- LocalWords:  nonterminal yy args Pragma dereference yyformat rhs docdir
+ LocalWords:  nonterminal yy args Pragma dereference yyformat rhs docdir bw
  LocalWords:  Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
  LocalWords:  Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
  LocalWords:  XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits

data/bison-default.css

@@ -0,0 +1,24 @@
+/* Default styling rules for Bison when doing terminal output.
+   Copyright (C) 2006-2007 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* This is an experimental feature.  The class names may change in the
+   future.  */
+
+.warning   { color: purple; }
+.error     { color: red; }
+.note      { color: cyan; }
+/* Semantic values in Bison's own parser traces.  */
+.value     { color: green; }

data/diagnostics.css

@@ -1,10 +0,0 @@
-/* CSS style for Bison's diagnostics.
-
-   This is an experimental feature.  The class names may change in the
-   future.  */
-
-.warning   { color: purple; }
-.error     { color: red; }
-.note      { color: cyan; }
-/* Semantic values in Bison's parser traces.  */
-.value     { color: green; }

data/local.mk

@@ -16,7 +16,7 @@
 
 dist_pkgdata_DATA =                             \
   data/README.md                                \
-  data/diagnostics.css
+  data/bison-default.css
 
 skeletonsdir = $(pkgdatadir)/skeletons
 dist_skeletons_DATA =                           \

src/complain.c

@@ -255,14 +255,13 @@ complain_init_color (void)
       || color_mode == color_html
       || (color_mode == color_tty && isatty (STDERR_FILENO)))
     {
-      style_file_prepare ("BISON_DIAGNOSTICS_STYLE", NULL,
-                          pkgdatadir (),
-                          "diagnostics.css");
+      style_file_prepare ("BISON_STYLE", NULL, pkgdatadir (),
+                          "bison-default.css");
       /* As a fallback, use the default in the current directory.  */
       struct stat statbuf;
       if ((style_file_name == NULL || stat (style_file_name, &statbuf) < 0)
-          && stat ("diagnostics.css", &statbuf) == 0)
-        style_file_name = "diagnostics.css";
+          && stat ("bison-default.css", &statbuf) == 0)
+        style_file_name = "bison-default.css";
     }
   else
     /* No styling.  */