Adds automake subdir-objects option

As of Automake 1.4, it starts to warning about the lack of utilization
of `subdir-objects' option, which will be default in the further
releases. Avoiding break stuff we are patching ModSecurity to support
such option when it still an option (Issue owasp-modsecurity#760).

configure.ac

@@ -14,7 +14,7 @@ AC_CONFIG_HEADERS([apache2/modsecurity_config_auto.h])
 AC_CONFIG_AUX_DIR([build])
 AC_PREFIX_DEFAULT([/usr/local/modsecurity])
 
-AM_INIT_AUTOMAKE([-Wall foreign])
+AM_INIT_AUTOMAKE([-Wall foreign subdir-objects])
 m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
 
 LT_PREREQ([2.2])