Makefiles should be able to invoke trunnel
I'd like the makefiles to be able to regenerate src/trunnel/.[ch] from src/trunnel/.trunnel as needed or on demand. This is a little nontrivial, since we'd need the following:
- A way to detect trunnel of the appropriate version.
- Actual meaningful version numbers on trunnel.
- A way to not require developers to have trunnel installed.
- Support for out-of-tree builds (which can get really hard with generated code, and even harder with optionally generated code).