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.

m4/.gitignore

@@ -5,6 +5,8 @@
 /asm-underscore.m4
 /assert.m4
 /calloc.m4
+/canonicalize.m4
+/clock_time.m4
 /close-stream.m4
 /close.m4
 /closeout.m4
@@ -36,19 +38,26 @@
 /frexpl.m4
 /fseterr.m4
 /fstat.m4
+/fsync.m4
 /getdtablesize.m4
+/gethrxtime.m4
 /getopt.m4
 /getprogname.m4
+/getrusage.m4
 /gettext.m4
+/gettime.m4
+/gettimeofday.m4
 /glibc2.m4
 /glibc21.m4
 /gnulib-cache.m4
 /gnulib-common.m4
 /gnulib-comp.m4
 /gnulib-tool.m4
+/host-cpu-c-abi.m4
 /iconv.m4
 /include_next.m4
 /intdiv0.m4
+/intl-thread-locale.m4
 /intl.m4
 /intldir.m4
 /intlmacosx.m4
@@ -70,12 +79,15 @@
 /lib-ld.m4
 /lib-link.m4
 /lib-prefix.m4
+/libtextstyle-optional.m4
+/libtextstyle.m4
 /libunistring-base.m4
 /limits-h.m4
 /localcharset.m4
 /locale-fr.m4
 /locale-ja.m4
 /locale-zh.m4
+/localtime-buffer.m4
 /lock.m4
 /longlong.m4
 /lstat.m4
@@ -117,9 +129,15 @@
 /quotearg.m4
 /raise.m4
 /rawmemchr.m4
+/readlink.m4
 /realloc.m4
+/relocatable-lib.m4
+/relocatable.m4
+/rename.m4
+/rmdir.m4
 /sched_h.m4
 /setenv.m4
+/sh-filename.m4
 /sig_atomic_t.m4
 /sigaction.m4
 /signal_h.m4
@@ -132,7 +150,9 @@
 /spawn_h.m4
 /sprintf-posix.m4
 /ssize_t.m4
+/stat-time.m4
 /stat.m4
+/std-gnu11.m4
 /stdbool.m4
 /stddef_h.m4
 /stdint.m4
@@ -148,12 +168,16 @@
 /strndup.m4
 /strnlen.m4
 /strverscmp.m4
+/sys_resource_h.m4
 /sys_socket_h.m4
 /sys_stat_h.m4
+/sys_time_h.m4
+/sys_times_h.m4
 /sys_types_h.m4
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 /threadlib.m4
 /time_h.m4
+/timespec.m4
 /uintmax_t.m4
 /unistd-safer.m4
 /unistd_h.m4
@@ -177,25 +201,3 @@
 /xalloc.m4
 /xsize.m4
 /xstrndup.m4
-/host-cpu-c-abi.m4
-/stat-time.m4
-/sys_time_h.m4
-/sys_times_h.m4
-/sh-filename.m4
-/clock_time.m4
-/gethrxtime.m4
-/getrusage.m4
-/gettime.m4
-/gettimeofday.m4
-/localtime-buffer.m4
-/sys_resource_h.m4
-/timespec.m4
-/intl-thread-locale.m4
-/canonicalize.m4
-/readlink.m4
-/relocatable-lib.m4
-/relocatable.m4
-/libtextstyle.m4
-/rename.m4
-/rmdir.m4
-/std-gnu11.m4