diagnostics: use libtextstyle for colored output

Bruno Haible released libtextstyle, a library for colored output based
on CSS.  Let's use it to generate colored diagnostics, provided
libtextstyle is available.

See
https://lists.gnu.org/archive/html/bug-gnulib/2019-01/msg00176.html
https://lists.gnu.org/archive/html/bison-patches/2019-02/msg00073.html
https://lists.gnu.org/archive/html/bison-patches/2019-02/msg00084.html
https://lists.gnu.org/archive/html/bison-patches/2019-03/msg00007.html

* bootstrap.conf (gnulib_modules): Use libtextstyle when possible.
* data/diagnostics.css: New.
* src/complain.c (begin_use_class, end_use_class, flush)
(severity_style, complain_init_color): New.
Use them.
* src/getargs.c (getargs_colors): New.
(getargs): Use it.
Skip --color and --style.
* src/location.h, src/location.c (location_print): Use a style.

* tests/bison.in: Force --color=yes when stderr is a tty.
* tests/local.at: Disable colors during the test suite.
* tests/input.at: Adjust expectations to the extra options passed on
the command line.

src/complain.h

@@ -24,6 +24,20 @@
 /* Sub-messages indent. */
 # define SUB_INDENT (4)
 
+/*---------------.
+| Error stream.  |
+`---------------*/
+
+/** Enable a style on \a out provided it's stderr.  */
+void begin_use_class (const char *style, FILE *out);
+
+/** Disable a style on \a out provided it's stderr.  */
+void end_use_class (const char *style, FILE *out);
+
+/** Flush \a out.  */
+void flush (FILE *out);
+
+
 /*-------------.
 | --warnings.  |
 `-------------*/
@@ -76,6 +90,9 @@ void warnings_argmatch (char *args);
 /** Initialize this module.  */
 void complain_init (void);
 
+/** Initialize support for colored messages.  */
+void complain_init_color (void);
+
 typedef enum
   {
     Wnone             = 0,       /**< Issue no warnings.  */