Move pcap function checks to configure.in.

Move pcap function checks to configure.in.

It was doing some of the function checks; move the remaining ones out of
AC_LBL_LIBPCAP(), which is now solely responsible for finding libpcap,
making sure it'll work, and setting compiler and linker flags
appropriately.  Checks for functions that were introduced in later
versions of libpcap, which we use if available, are all done in
configure.in.