In a fakeroot-environment: "ocamlfind: Cannot mkdir /usr/lib/ocaml/netsys: Permission denied"

I tried installing the package for an AUR-package (Arch User Repository). Compilation works.

Installation fails:

==> Entering fakeroot environment...
==> Starting package()...
for pkg in netsys netshm netstring netunidata equeue shell rpc-generator rpc rpc-auth-local netclient netcgi2 netplex netcgi2-plex netcamlbox netmulticore nettls-gnutls netgss-system; do \
        ( cd src/$pkg && make -f Makefile.pre install ) || exit; \
done
make[1]: Entering directory '/home/oliver/AUR4/ocamlnet/src/lib-ocamlnet3-ocamlnet-4.1.9/code/src/netsys'
make[2]: Entering directory '/home/oliver/AUR4/ocamlnet/src/lib-ocamlnet3-ocamlnet-4.1.9/code/src/netsys'
files=`../../tools/collect_files *.mli *.cmi *.cma *.cmxa *.cmxs *.a dll* META netsys_oothr.cmo netsys_oothr_mt_init.cmo netsys_oothr_mt_init.cmx netsys_oothr_mt_init.o netsys_oothr_mt_init.p.cmx netsys_oothr_mt_init.p.o netsys_gprof_init.p.cmx netsys_gprof_init.p.o netsys_c_gprof.o netsys_c_event.h netsys_c_outofheap.o` && \
ocamlfind install netsys $files
ocamlfind: Cannot mkdir /usr/lib/ocaml/netsys: Permission denied
make[2]: *** [../../Makefile.rules:179: install-findlib] Error 2
make[2]: Leaving directory '/home/oliver/AUR4/ocamlnet/src/lib-ocamlnet3-ocamlnet-4.1.9/code/src/netsys'
make[1]: *** [../../Makefile.rules:164: install] Error 2
make[1]: Leaving directory '/home/oliver/AUR4/ocamlnet/src/lib-ocamlnet3-ocamlnet-4.1.9/code/src/netsys'
make: *** [Makefile:58: install] Error 2
==> ERROR: A failure occurred in package().
    Aborting...

The dir can't be created, even though the installation is run in a fakeroot environment. I wonder how that can happen. Ideas for a solution?