Initial work to support out-of-tree builds
```
./autogen.sh
mkdir build
cd build
../configure
make
make check
```
IMPORTANT: `autogen.sh` doesn't call `configure` automatically anymore!!
You have to do: `./autogen.sh && ./configure --$OPTIONS`.
A little bit annoying but the pattern `autogen && configure && make` is
very common on Linux.
Known issues:
* `make doc` doesn't work in out-of-tree builds, yet
* Windows/MinGW/DPDK (out-of-tree) builds have not been tested, so it is unlikely they work
See: #2992
We don't need specific targets to reproduce fuzzing issues.
After all, calling `./fuzz/fuzz_process_packet_with_main $ARTIFACT_FILE`
is equivalento to `./fuzz/fuzz_process_packet $ARTIFACT_FILE`