* doc/autoconf.texi (Particular Programs): AC_PROG_MKDIR_P now

* doc/autoconf.texi (Particular Programs): AC_PROG_MKDIR_P now
sets MKDIR_P, not mkdir_p, to avoid collisions with Automake.
Warn about obsolete install-sh files.  Remove stray sentence
fragment and fix cross reference.
* lib/autoconf/programs.m4 (AC_PROG_INSTALL): Don't insist on
install -d; this undoes the 2006-05-10 change.
(MKDIR_P): Mark with AN_MAKEVAR.
(AC_PROG_MKDIR_P): Fall back on $ac_install_sh, not $INSTALL, so
that we don't require $INSTALL to be thread-safe.  Move comments
out of generated code.  Require AC_CONFIG_AUX_DIR_DEFAULT instead
of AC_PROG_INSTALL.  Output a message saying that we're checking
mkdir -p.  Set MKDIR_P rather than mkdir_p.  Do special magic for
MKDIR_P instead of AC_SUBST.
* lib/autoconf/status.m4 (_AC_OUTPUT_FILE, _AC_OUTPUT_CONFIG_STATUS):
Special magic for MKDIR_P, too.
* lib/m4sugar/m4sh.m4 (AS_MKDIR_P): Remove comment that defeated
a dnl.
* tests/local.at (AT_CHECK_ENV): mkdir_p -> MKDIR_P.