add cscope Make rule

Author: chrisbra

src/Makefile

@@ -2154,6 +2154,15 @@ notags:
 tags TAGS: notags
 	$(TAGPRG) $(TAGS_FILES)
 
+# Build the cscope database.
+# This may search more files than necessary.
+.PHONY: cscope csclean
+csclean:
+	-rm -vf cscope.out
+cscope.out:
+	cscope -bv ./*.[ch] ./*.cpp ./if_perl.xs auto/*.h auto/pathdef.c proto/*.pro
+cscope: csclean cscope.out  ;
+
 # Make a highlight file for types.  Requires Exuberant ctags and awk
 types: types.vim
 types.vim: $(TAGS_FILES)