-FINDRULES := ! '(' '(' -name '[.-]*' -o -name '*[][*?:|$$%\#\\; ]*' ')' -a -prune ')'
-FINDINCLUDERULES := $(FINDRULES)
+EXTRAFINDRULES :=
+EXTRAFINDINCLUDERULES :=
+FINDRULES := ! '(' '(' -name '[.-]*' -o -name '*[][*?:|$$%\#\\; ]*' ')' -a -prune ')'$(if $(EXTRAFINDRULES), -a $(EXTRAFINDRULES),)
+FINDINCLUDERULES := $(FINDRULES)$(if $(EXTRAFINDINCLUDERULES), -a $(EXTRAFINDINCLUDERULES),)