Additional locations for Clownfish include dirs

The environment variable CLOWNFISH_INCLUDE can be set to a
colon-separated list of directories which cfc will use as include
directories. For now, this feature is only available for the C bindings.

The C bindings will also add the following system-wide include
directories:

    * /usr/local/share/clownfish/include
    * /usr/share/clownfish/include

If CLOWNFISH_INCLUDE is set, the system-wide directories will not be used
to avoid errors when a parcel is found in multiple locations.
Rework installation script of C library

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