diagnostics: support --color=html

Based on a message from Bruno Haible.
https://git.savannah.gnu.org/gitweb/?p=gettext.git;a=commitdiff;h=fe18e92743b7226791a5f28d7c786941a1bf8cc9

This does not generate proper HTML: special characters are not escaped
for instance.  This is a hidden feature meant for Bison developers,
not end users.

* src/complain.c (complain_init_color): Support --color=html.

src/complain.c

@@ -252,6 +252,7 @@ complain_init_color (void)
 {
 #if HAVE_LIBTEXTSTYLE
   if (color_mode == color_yes
+      || color_mode == color_html
       || (color_mode == color_tty && isatty (STDERR_FILENO)))
     {
       style_file_prepare ("BISON_DIAGNOSTICS_STYLE", NULL,
@@ -268,9 +269,25 @@ complain_init_color (void)
     style_file_name = NULL;
 #endif
 
+  /* Workaround clang's warning (starting at Clang 3.5) about the stub
+     code of html_styled_ostream_create:
+
+     | src/complain.c:274:7: error: code will never be executed [-Werror,-Wunreachable-code]
+     |     ? html_styled_ostream_create (file_ostream_create (stderr),
+     |       ^~~~~~~~~~~~~~~~~~~~~~~~~~ */
+#if defined __clang__
+# pragma clang diagnostic push
+# pragma clang diagnostic ignored "-Wunreachable-code"
+#endif
   errstream =
-    styled_ostream_create (STDERR_FILENO, "(stderr)", TTYCTL_AUTO,
+    color_mode == color_html
-                           style_file_name);
+    ? html_styled_ostream_create (file_ostream_create (stderr),
+                                  style_file_name)
+    : styled_ostream_create (STDERR_FILENO, "(stderr)", TTYCTL_AUTO,
+                             style_file_name);
+#if defined __clang__
+# pragma clang diagnostic pop
+#endif
 }
 
 void