build: fix dragonflybsd autotools build

Fixes multiple definition linker errors due to what seems to be multiple
inclusions of src/unix/freebsd.c and src/unix/kqueue.c.

Fixes #72, refs joyent/libuv#1576.

PR-URL: https://github.com/libuv/libuv/issues/72
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>