automake: use AC_PROG_MKDIR_P instead of AM_PROG_MKDIR_P

autoreconf emits the warning below. It can be resolved by following the
advice provided by the warning. Though this is just a warning, it is fatal
on Ubuntu because warnings are treated as errors.
 See: https://bugs.launchpad.net/ubuntu/+source/partclone/+bug/1283294

Warnings from autoreconf:
  configure.ac:4: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and its use is discouraged.
  configure.ac:4: You should use the Autoconf-provided 'AC_PROG_MKDIR_P' macro instead,
  configure.ac:4: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your Makefile.am files.

m4/po.m4

@@ -24,7 +24,7 @@ AC_DEFUN([AM_PO_SUBDIRS],
 [
   AC_REQUIRE([AC_PROG_MAKE_SET])dnl
   AC_REQUIRE([AC_PROG_INSTALL])dnl
-  AC_REQUIRE([AM_PROG_MKDIR_P])dnl defined by automake
+  AC_REQUIRE([AC_PROG_MKDIR_P])dnl defined by automake
   AC_REQUIRE([AM_NLS])dnl
 
   dnl Perform the following tests also if --disable-nls has been given,