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.

lib/.gitignore

@@ -319,3 +319,5 @@
 /rmdir.c
 /same-inode.h
 /assure.h
+/fsync.c
+/textstyle.in.h