This patch adds a --libdir option to opam-installer so it installs the plugin
in the specified directory rather than in the default one (ocaml's directory in
the store, which is forbidden).
diff -aur package.pristine/Makefile package.new/Makefile
--- package.pristine/Makefile	2016-02-06 01:55:14.650150309 +0100
+++ package.new/Makefile	2016-02-06 01:57:56.012174364 +0100
@@ -29,26 +29,26 @@
 	ocaml -I js-utils js-utils/gen_install.ml

 install: $(NAME).install
-	opam-installer -i --prefix $(PREFIX) $(NAME).install
+	opam-installer -i --prefix $(PREFIX) --libdir $(LIBDIR) $(NAME).install

 uninstall: $(NAME).install
-	opam-installer -u --prefix $(PREFIX) $(NAME).install
+	opam-installer -u --prefix $(PREFIX) --libdir $(LIBDIR) $(NAME).install

 reinstall: $(NAME).install
-	opam-installer -u --prefix $(PREFIX) $(NAME).install &> /dev/null || true
-	opam-installer -i --prefix $(PREFIX) $(NAME).install
+	opam-installer -u --prefix $(PREFIX) --libdir $(LIBDIR) $(NAME).install &> /dev/null || true
+	opam-installer -i --prefix $(PREFIX) --libdir $(LIBDIR) $(NAME).install

 bin.tar.gz: $(NAME).install
 	rm -rf _install
 	mkdir _install
-	opam-installer -i --prefix _install $(NAME).install
+	opam-installer -i --prefix _install --libdir $(LIBDIR) $(NAME).install
 	tar czf bin.tar.gz -C _install .
 	rm -rf _install

 bin.lzo: $(NAME).install
 	rm -rf _install
 	mkdir _install
-	opam-installer -i --prefix _install $(NAME).install
+	opam-installer -i --prefix _install --libdir $(LIBDIR) $(NAME).install
 	cd _install && lzop -1 -P -o ../bin.lzo `find . -type f`
 	rm -rf _install