Update after gnulib changed.
* configure.ac (AC_PREREQ): Bump minimum Autoconf version to 2.64.
* gettext-runtime/configure.ac (AC_PREREQ): Likewise.
* gettext-runtime/libasprintf/configure.ac (AC_PREREQ): Likewise.
* libtextstyle/configure.ac (AC_PREREQ): Likewise.
* gettext-tools/configure.ac (AC_PREREQ): Likewise.
* gettext-tools/examples/configure.ac (AC_PREREQ): Likewise.
