Update for autotools 1.14

Fixes:

configure.ac:30: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and will soon be removed.
configure.ac:30: You should use the Autoconf-provided 'AC_PROG_MKDIR_P' macro instead,
configure.ac:30: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your Makefile.am files.

Ref:
http://lists.gnu.org/archive/html/automake/2012-11/msg00074.html