$ opam install why
The following actions will be performed:
 - install why.2.33
1 to install | 0 to reinstall | 0 to upgrade | 0 to downgrade | 0 to remove

=-=-= Installing why.2.33 =-=-=
The archive for why.2.33 is in the local cache.
Extracting /usr/local/stow/opam-root/archives/why.2.33+opam.tar.gz.
Applying opam.patch.
Applying warn-error.patch.
Applying configure.Fluorine-version.
Building why.2.33:
  autoconf
  ./configure --enable-verbosemake OCAMLGRAPHLIB=/usr/local/stow/opam-root/system/lib/ocamlgraph --prefix /usr/local/stow/opam-root/system --sbindir=/usr/local/stow/opam-root/system/lib/why/sbin --libexecdir=/usr/local/stow/opam-root/system/lib/why/libexec --sysconfdir=/usr/local/stow/opam-root/system/lib/why/etc --sharedstatedir=/usr/local/stow/opam-root/system/lib/why/com --localstatedir=/usr/local/stow/opam-root/system/lib/why/var --libdir=/usr/local/stow/opam-root/system/lib/why/lib --includedir=/usr/local/stow/opam-root/system/lib/why/include --datarootdir=/usr/local/stow/opam-root/system/lib/why/share
  make
  make install
The compilation of why.2.33 failed.
Uninstalling why.2.33.


==== ERROR [while installing why.2.33] ====
# opam-version    1.0.0
# os              linux
# command         ./configure --enable-verbosemake OCAMLGRAPHLIB=/usr/local/stow/opam-root/system/lib/ocamlgraph --prefix /usr/local/stow/opam-root/system --sbindir=/usr/local/stow/opam-root/system/lib/why/sbin --libexecdir=/usr/local/stow/opam-root/system/lib/why/libexec --sysconfdir=/usr/local/stow/opam-root/system/lib/why/etc --sharedstatedir=/usr/local/stow/opam-root/system/lib/why/com --localstatedir=/usr/local/stow/opam-root/system/lib/why/var --libdir=/usr/local/stow/opam-root/system/lib/why/lib --includedir=/usr/local/stow/opam-root/system/lib/why/include --datarootdir=/usr/local/stow/opam-root/system/lib/why/share
# path            /usr/local/stow/opam-root/system/build/why.2.33
# exit-code       1
# env-file        /usr/local/stow/opam-root/system/build/why.2.33/why-548b09.env
# stdout-file     /usr/local/stow/opam-root/system/build/why.2.33/why-548b09.out
# stderr-file     /usr/local/stow/opam-root/system/build/why.2.33/why-548b09.err
### stdout ###
...[truncated]
checking ocamlc.opt version... ok
checking for ocamldep... ocamldep
checking for ocamldep.opt... ocamldep.opt
checking for ocamllex... ocamllex
checking for ocamllex.opt... ocamllex.opt
checking for ocamlyacc... ocamlyacc
checking for ocamldoc... ocamldoc
checking for ocamldoc.opt... ocamldoc.opt
checking for /usr/local/stow/ocaml-4.00.1/lib/ocaml/ocamlgraph/graph.cmi... no
checking for ocamlgraph/src/sig.mli... no
### stderr ###
configure: error: Cannot find ocamlgraph library. Please install the *libocamlgraph-ocaml-dev* Debian package - or use the GODI caml package system *http://godi.ocaml-programming.de/* - or compile from sources *http://ocamlgraph.lri.fr/*

'opam install why' failed.