Replace AM_PROG_MKDIR obsolete macro with AC_PROG_MKDIR_P

The bootstrap script is complaining with a deprecation warning [1] for
the AM_PROG_MKDIR_P Automake macro.

Relevant bootstrap output:

configure.ac:13: warning: The 'AM_PROG_MKDIR_P' macro is deprecated,
                 and will soon be removed.
configure.ac:13: You should use the Autoconf-provided 'AC_PROG_MKDIR_P'
                 macro instead,
configure.ac:13: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your
                 Makefile.am files.

Fix this by updating configure.ac with the proposed AC_PROG_MKDIR_P macro.

[1] - https://www.gnu.org/software/automake/manual/html_node/Obsolete-Macros.html

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>