Enables mergemaster to use system include files from the source
directory (read /usr/src) prior to use their version from the system.

It is useful when some file from /usr/share/mk gained some functionality
and some Makefile(s) depend on this new stuff.

--- usr.sbin/mergemaster.orig	2008-09-27 14:24:14.000000000 +0400
+++ usr.sbin/mergemaster	2008-09-27 14:37:58.000000000 +0400
@@ -414,6 +414,11 @@
 #
 SOURCEDIR=${SOURCEDIR:-/usr/src/etc}
 
+# New versions of system makefiles should take precedence
+# over the default one.
+#
+MAKE_FLAGS="-m ${SOURCEDIR%etc}share/mk -m /usr/share/mk"
+
 # Check DESTDIR against the mergemaster mtree database to see what
 # files the user changed from the reference files.
 #
@@ -552,13 +557,13 @@
       case "${DESTDIR}" in
       '') ;;
       *)
-      make DESTDIR=${DESTDIR} ${ARCHSTRING} distrib-dirs
+      make ${MAKE_FLAGS} DESTDIR=${DESTDIR} ${ARCHSTRING} distrib-dirs
         ;;
       esac
-      make DESTDIR=${TEMPROOT} ${ARCHSTRING} distrib-dirs &&
-      MAKEOBJDIRPREFIX=${TEMPROOT}/usr/obj make ${ARCHSTRING} obj &&
-      MAKEOBJDIRPREFIX=${TEMPROOT}/usr/obj make ${ARCHSTRING} all &&
-      MAKEOBJDIRPREFIX=${TEMPROOT}/usr/obj make ${ARCHSTRING} \
+      make ${MAKE_FLAGS} DESTDIR=${TEMPROOT} ${ARCHSTRING} distrib-dirs &&
+      MAKEOBJDIRPREFIX=${TEMPROOT}/usr/obj make ${MAKE_FLAGS} ${ARCHSTRING} obj &&
+      MAKEOBJDIRPREFIX=${TEMPROOT}/usr/obj make ${MAKE_FLAGS} ${ARCHSTRING} all &&
+      MAKEOBJDIRPREFIX=${TEMPROOT}/usr/obj make ${MAKE_FLAGS} ${ARCHSTRING} \
 	  DESTDIR=${TEMPROOT} distribution;} ||
     { echo '';
      echo "  *** FATAL ERROR: Cannot 'cd' to ${SOURCEDIR} and install files to";