Fix build with automake >= 1.12.1

Fix build with automake >= 1.12.1

mkdir_p is deprecated since automake 1.12.1 (see automake commit
7a1eb9ff9027929687f12905e131f6fda3fa6d0c).

MKDIR_P should be used instead of mkdir_p.
This is available since autoconf 2.59d (2006-06-05), by calling
AC_PROG_MKDIR_P.

The mkdir_p workaround was not working anyway for out-of-tree builds,
since the ../mkinstalldirs path would be incorrect.