AC_PROG_MKDIR_P: Fix detection of busybox mkdir.

AC_PROG_MKDIR_P: Fix detection of busybox mkdir.

Some versions of BusyBox mkdir respond to ‘mkdir --version’ by
printing “mkdir: unrecognized option: --version” to stderr, and then
the version message we’re looking for, also to stderr.  Adjust the
pattern matching in AC_PROG_MKDIR_P to account for this.

Problem reported by Valery Ushakov.
Resolves <https://savannah.gnu.org/support/?110971>.

* lib/autoconf/programs.m4 (AC_PROG_MKDIR_P): Fix detection of busybox mkdir.