Declare a private dependency on libdl.

The new linker requires that the consumers of a library link against
the libraries that that library depends on, which is supported by
Libs.private.

Fixes #16
Fix a typo in the Makefile.am.

Fixes #15 (and make dist when built out of tree)
Add missing make dist of gen_dispatch.py.

Fixes distcheck when built out of tree (not that I'm using make dist
for distributing releases, currently).