diagnostics: use gnulib's libtextstyle-optional

Bruno Haible just added a default implementation of libtextstyle's
interface when the library is not available.
https://lists.gnu.org/archive/html/bison-patches/2019-03/msg00025.html

* gnulib: Update.
* bootstrap.conf: Replace libtextstyle with libtextstyle-optional.
* src/complain.c, src/getargs.c: Remove now useless cpp guards.

data/diagnostics.css

@@ -1,3 +1,8 @@
-.warning   { color : purple; }
-.error     { color : red; }
-.note      { color : cyan; }
+/* 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; }