Add site.{autodep,gendirdeps}.mk

Allow for site customization to minimize churn in share/mk

Reviewed by:	stevek
Sponsored by:	Juniper Networks, Inc.
Differential Revision:	https://reviews.freebsd.org/D37895

share/mk/local.autodep.mk

@@ -27,3 +27,5 @@ CLEANFILES+= .depend
 
 .cc.cpp-out: .NOMETA
 	@${CXX} -E ${CXXFLAGS} ${.IMPSRC} | grep -v '^[[:space:]]*$$'
+
+.-include <site.autodep.mk>

share/mk/local.gendirdeps.mk

@@ -70,3 +70,5 @@ GENDIRDEPS_FILTER+= ${GENDIRDEPS_FILTER_VARS:@v@S,/${$v}/,/_{${v}}/,@:NS,//,*:u}
 
 # avoid churn for now
 LOCAL_DEPENDS_GUARD= _{DEP_RELDIR} == _{_DEP_RELDIR}
+
+.-include <site.gendirdeps.mk>