Support extensions written in C

Currently, all Lucy extensions that contain C code must be shipped with the Lucy source tree. In order to support external C-based extensions, Clownfish must be able to compile .cfh files that extend classes from a separate source tree. As discussed on the mailing list, a solution could look like this:

• Install the .cfh files together with Lucy.
• When building the extension, rebuild the complete Clownfish hierarchy together with the new classes.
• Generate C headers, regenerating the headers for Lucy's core classes.
• Compile the extension.