gnulib: update for fix of fprintf-posix, which we'll use soon.

* etc/prefix-gnulib-mk (prefix): Adjust regex for make file
targets so that gnulib's new arg-nonnull.h and link-warning.h
are matched.
* gnulib: Update.

m4/.gitignore

@@ -11,6 +11,8 @@
 /error.m4
 /exitfail.m4
 /extensions.m4
+/fcntl_h.m4
+/fopen.m4
 /getopt.m4
 /getpagesize.m4
 /gettext.m4
@@ -55,6 +57,7 @@
 /stdint.m4
 /stdint_h.m4
 /stdio-safer.m4
+/stdio_h.m4
 /stdlib_h.m4
 /stpcpy.m4
 /strerror.m4