opam pin add piqilib ~/workspace/piqi -y

...

piqilib needs to be installed.
The following actions will be performed:
 - install   piqilib.0.6.12*
=== 1 to install ===

=-=- Synchronizing package archives -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[piqilib.0.6.12] Synchronizing with /home/xyz/workspace/piqi

=-=- Installing packages =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Building piqilib.0.6.12:
  ./configure --prefix /home/xyz/.opam/4.02.1
  make
  make ocaml
  make install
  make ocaml-install
[ERROR] The compilation of piqilib.0.6.12 failed.
Removing piqilib.0.6.12.
  rm -f /home/xyz/.opam/4.02.1/bin/piqi
  ocamlfind remove piqilib


#=== ERROR while installing piqilib.0.6.12 ====================================#
# opam-version 1.2.0
# os           linux
# command      make
# path         /home/xyz/.opam/4.02.1/build/piqilib.0.6.12
# compiler     4.02.1
# exit-code    2
# env-file     /home/xyz/.opam/4.02.1/build/piqilib.0.6.12/piqilib-28358-58c514.env
# stdout-file  /home/xyz/.opam/4.02.1/build/piqilib.0.6.12/piqilib-28358-58c514.out
# stderr-file  /home/xyz/.opam/4.02.1/build/piqilib.0.6.12/piqilib-28358-58c514.err
### stdout ###
# ...[truncated]
# ocamlfind ocamlopt -package ulex,easy-format,xmlm,base64,bytes -c piqloc.ml
# ocamlfind ocamlopt -package ulex,easy-format,xmlm,base64,bytes -c piq_ast.ml
# ocamlfind ocamlopt -package ulex,easy-format,xmlm,base64,bytes -c piqi_impl_piqi.ml
# ocamlfind ocamlopt -package ulex,easy-format,xmlm,base64,bytes -c piqi_boot.ml
# ocamlfind ocamlopt -package ulex,easy-format,xmlm,base64,bytes -c piqi_c.ml
# ocamlfind ocamlopt -package ulex,easy-format,xmlm,base64,bytes -c piqi_config.ml
# ocamlfind ocamlopt -package ulex,easy-format,xmlm,base64,bytes -c piqi_iolist.ml
# ocamlfind ocamlopt -package ulex,easy-format,xmlm,base64,bytes -c -pp "camlp4o -I /home/xyz/.opam/4.02.1/lib/optcomp optcomp.cma " piqi_common.ml
# make[2]: Leaving directory `/home/xyz/.opam/4.02.1/build/piqilib.0.6.12/piqilib'
# make[1]: Leaving directory `/home/xyz/.opam/4.02.1/build/piqilib.0.6.12/piqilib'
### stderr ###
# ...[truncated]
# make[2]: *** Deleting file `._d/piqi_common.d'
# File "piqi_common.ml", line 327, characters 2-6:
# Parse error: [semi] expected after [str_item] (in [implem])
# File "piqi_common.ml", line 1:
# Error: Error while running external preprocessor
# Command line: camlp4o -I /home/xyz/.opam/4.02.1/lib/optcomp optcomp.cma  'piqi_common.ml' > /tmp/ocamlppdaa372