build: Remove a .m4 file that duplicates gnulib functionality.
* gettext-runtime/m4/printf-posix.m4: Remove file.
* gettext-runtime/m4/Makefile.am (EXTRA_DIST): Remove it.
* gettext-runtime/intl/configure.ac: Use gl_PRINTF_POSITIONS instead of
gt_PRINTF_POSIX.
* gettext-runtime/libasprintf/configure.ac: Likewise.
