Use Autoconf macros AC_PROG_MKDIR_P and MKDIR_P

configure complains AM_PROG_MKDIR_P and MKDIR_P are obsolete.
configure.ac:44: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and
its use is discouraged.
configure.ac:44: You should use the Autoconf-provided 'AC_PROG_MKDIR_P'
macro instead,
configure.ac:44: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your
Makefile.am files.

Automake manual says:
From Automake 1.8 to 1.9.6 AM_PROG_MKDIR_P used to define the output variable
mkdir_p to one of mkdir -p, install-sh -d, or mkinstalldirs

Nowadays Autoconf provides a similar functionality with AC_PROG_MKDIR_P.
Automake manual advises to switch ASAP to the more modern Autoconf-provided
interface instead; both the macro and the variable might be removed in a
future major Automake release.

AC_PROG_MKDIR_P will set output variable MKDIR_P.

And kimchi already contains install-sh under build-aux.

REF:
http://www.gnu.org/software/automake/manual/automake.html#Obsolete-Macros
http://www.gnu.org/software/autoconf/manual/autoconf.html#index-AC_005fPROG_005fMKDIR_005fP-277

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>