Hi,

I had the same error using homebrew. For some reason, sqlite3.pc 
(pkgconfig) wasn't in usual directory.
I just manually linked it to /usr/lib/pkgconfig:
     ln -s /usr/local/Cellar/sqlite/3.7.15.2/lib/pkgconfig/sqlite3.pc 
/usr/lib/pkgconfig/sqlite3.pc
Then "opam install sqlite3-ocaml" ran fine.

Sqlite3 was the only package having this problem.

On 28/02/2013 04:14, manu delab wrote:
> Hello,
>
> I get the following error when trying to install sqlite3-ocaml:
>
> $ opam install sqlite3-ocaml
> The following actions will be performed:
>  - install sqlite3-ocaml.2.0.4
> 1 to install | 0 to reinstall | 0 to upgrade | 0 to downgrade | 0 to 
> remove
>
> =-=-= Installing sqlite3-ocaml.2.0.4 =-=-=
> The archive for sqlite3-ocaml.2.0.4 is in the local cache.
> Extracting /Users/manu/.opam/archives/sqlite3-ocaml.2.0.4+opam.tar.gz.
> Building sqlite3-ocaml.2.0.4:
>   ocaml setup.ml <http://setup.ml> -configure
>   ocaml setup.ml <http://setup.ml> -build
>   ocaml setup.ml <http://setup.ml> -install
> The compilation of sqlite3-ocaml.2.0.4 failed.
> Uninstalling sqlite3-ocaml.2.0.4:
>   ocamlfind remove sqlite3
>
>
> ==== ERROR [while installing sqlite3-ocaml.2.0.4] ====
> # opam-version    0.9.4
> # os              darwin
> # command         ocaml setup.ml <http://setup.ml> -build
> # path  /Users/manu/.opam/system/build/sqlite3-ocaml.2.0.4
> # exit-code       1
> # env-file 
>  /Users/manu/.opam/system/build/sqlite3-ocaml.2.0.4/sqlite3-ocaml-1872ec.env
> # stdout-file 
> /Users/manu/.opam/system/build/sqlite3-ocaml.2.0.4/sqlite3-ocaml-1872ec.out
> # stderr-file 
> /Users/manu/.opam/system/build/sqlite3-ocaml.2.0.4/sqlite3-ocaml-1872ec.err
> ### stdout ###
> /opt/local/bin/ocamlopt.opt -I /opt/local/lib/ocaml/ocamlbuild 
> unix.cmxa /opt/local/lib/ocaml/ocamlbuild/ocamlbuildlib.cmxa 
> myocamlbuild.ml <http://myocamlbuild.ml> 
> /opt/local/lib/ocaml/ocamlbuild/ocamlbuild.cmx -o myocamlbuild
> Failure: pkg-config failed for cflags.
> ### stderr ###
> Package sqlite3 was not found in the pkg-config search path.
> Perhaps you should add the directory containing `sqlite3.pc'
> to the PKG_CONFIG_PATH environment variable
> No package 'sqlite3' found
> E: Failure("Command ''/opt/local/bin/ocamlbuild' 
> lib/libsqlite3_stubs.a lib/dllsqlite3_stubs.so lib/sqlite3.cma 
> lib/sqlite3.cmxa lib/sqlite3.a lib/sqlite3.cmxs -tag debug' terminated 
> with error code 2")
>
> 'opam install sqlite3-ocaml' failed.
>
> I am on OSX and I have the following installed with Macports:
>   sqlite3 @3.7.10_0
>   sqlite3 @3.7.11_0
>   sqlite3 @3.7.12.1_0
>   sqlite3 @3.7.13_0+universal
>   sqlite3 @3.7.14_1+universal
>   sqlite3 @3.7.14.1_0+universal
>   sqlite3 @3.7.15.2_0
>   sqlite3 @3.7.15.2_0+universal (active)
>
> Any idea ? Thanks
>
>