Use AC_PATH_TOOL, not AC_PATH_PROG, to find pcap-config.

The PROG variant does not take into consideration cross-compilation, so
it always looks for "pcap-config" which finds the host script.  So use
the TOOL variant instead which first searches for the cross-prefixed
pcap-config before falling back to the bare "pcap-config".

Reviewed-By: Guy Harris <[email protected]>