configure.ac: add AM_INIT_AUTOMAKE

This fixes installation where in po/Makefile @mkdir_p@ was not
defined and caused po/*gmo translations installation failure

Author: 01micko

configure.ac

@@ -8,6 +8,7 @@ AC_CONFIG_SRCDIR([src])
 AC_CONFIG_SRCDIR([po])
 AC_CONFIG_HEADERS([config.h])
 AC_LANG(C)
+AM_INIT_AUTOMAKE
 
 AC_PROG_CC
 AC_PROG_CPP