This makes running wireshark (or more specifically, dumpcap) as root a
bit more secure. From <wireshark-1.11.2>/doc/README.packaging:
The "--with-libcap" option is only useful when dumpcap is installed
setuid. If it is enabled dumpcap will try to drop any setuid privileges
it may have while retaining the CAP_NET_ADMIN and CAP_NET_RAW
capabilities. It is enabled by default, if the Linux capabilities
library (on which it depends) is found.
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