Add setup_clownfish_env.sh

This shell script sets up the necessary environment variables to build
Lucy with an uninstalled Clownfish source tree. Usage:

    . devel/bin/setup_clownfish_env.sh path_to_clownfish_source
Build the C bindings with an external Clownfish installation
Add a TODO comment
Regenerate charmonizer.c
Rework installation script of C library

* Install Clownfish headers.
* Don't install C headers. They can be generated with cfc from the
  Clownfish headers.
* Absolutify prefix.
* Add quotes in case prefix contains spaces.
* Rework pkg-config file.