Be sure "make" understands the "doc" target is phony.
diff --git a/Doc/Makefile b/Doc/Makefile
index 7487f15..b3e281f 100644
--- a/Doc/Makefile
+++ b/Doc/Makefile
@@ -61,7 +61,7 @@
 
 # These must be declared phony since there
 # are directories with matching names:
-.PHONY: api ext lib mac ref tut
+.PHONY: api doc ext lib mac ref tut
 .PHONY: html info