Start using config.h, and use it for HAVE_PCAP_NEXT_EX.

In #110, @vodik reported that HAVE_PCAP_NEXT_EX was unused. This
changeset does the following:

* Remove uint8_t and friends from configure.ac because the resulting
  defines (`define uint8_t unsigned char`) conflict with typedefs from
  `netinet/in.h`.
* Include `config.h` from `prepare_pcap.c`.
* Fix so HAVE_PCAP_NEXT_EX is defined in config.h if available.

TODO: We should move HAVE_EPOLL from the Makefile.am CFLAGS addition to
config.h inclusion as well.