Opam does not find homebrew-installed packages

I'm trying to install the chamo package from the opam
repository. The installations fails (see error message below). 

Now the packages that chamo does not find are indeed present
on my computer, but it would seem that opam does not see them
because (as they were installed by Homebrew) they are
located at /usr/local/Cellar rather than /usr/local.

Which environment variable should I change to make those
packages visible to opam ? Is it PKG_CONFIG_PATH as the error
message suggests ? How should I do it ?

Any help appreciated.

=-=- Error report -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
The following actions were aborted
  ∗  install chamo          2.03  
  ∗  install lablgtk        2.18.3
  ∗  install lablgtk-extras 1.5   
The following actions failed
  ∗  install conf-glade         2    
  ∗  install conf-gtksourceview 2    
  ∗  install pcre               7.1.5
No changes have been performed

=-=- conf-gtksourceview.2 troubleshooting -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
=> This package requires gtksourceview 2.0 development packages installed on your system
=> On OSX, install the `gtksourceview' and `libxml2' packages and define PKG_CONFIG_PATH:
      
           export PKG_CONFIG_PATH=/usr/X11/lib/pkgconfig/:/usr/local/lib/pkgconfig:/usr/local/opt/libxml2/lib/pkgconfig/