Fix automake warning message

[czchen]

Automake version 1:1.13.3-1.1ubuntu2 shows the following warning messages.

configure.ac:23: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and its use is discouraged.
configure.ac:23: You should use the Autoconf-provided 'AC_PROG_MKDIR_P' macro instead,
configure.ac:23: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your Makefile.am files.
src/Makefile.am:5: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')

[jserv]

Automake 1.14 works perfectly to me. Should we migrate?