#33 Fix logic to find pcap.h

[Moriyoshi Koizumi]

As the root for include paths isn't necessarily the filesystem's root directory (e.g. cross-build environments, --sysroot), and it's the preprocessor that knows the actual include paths, searching header files by testing existence on the filesystem paths doesn't always work. Actually configure fails on Mac OS X because non-essential header files now reside under the platform bundles and not /usr/include.

The same applies to bpf.h. Since it takes for granted that bpf.h is included from pcap.h, it should suffice to include pcap.h to test the constants from bpf.h .