Fix autotools

Had some problems building sipp with recent autotools (archlinux). Specifically the cached scripts weren't building `prepare_pcap.c` (something about nothing to satisfy `sipp-preprate_pcap.c`). Calling `autoreconf` manually then broke the `AX_HAVE_EPOLL` macros.

This merge request drops all automatically generated files and adds a `autogen.sh` script instead.