--- Makefile.orig
+++ Makefile
@@ -101,6 +101,7 @@
 
 OPAMINSTALLER_FLAGS = --prefix "$(call CYGPATH,$(DESTDIR)$(prefix))"
 OPAMINSTALLER_FLAGS += --mandir "$(call CYGPATH,$(DESTDIR)$(mandir))"
+OPAMINSTALLER_FLAGS += --docdir "$(call CYGPATH,$(DESTDIR)$(prefix)/share/doc)"
 
 # With ocamlfind, prefer to install to the standard directory rather
 # than $(prefix) if there are no overrides