build: Make ccache opt-in rather than opt-out

Using ccache by default for compilation if it is available on the system
might confuse people and in rare cases also lead to unexpected results.
Thus, from now on the use of ccache for compilation has to explicitely
specified by setting the CCACHE make variable to the name/path of the
ccache binary, i.e. `make CCACHE=ccache'

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
netsniff-ng: Combine redundant pcap file rotation logic into function

The code to create the next pcap dump file is duplicated for the
HAVE_TPACKET3 and !HAVE_TPACKET3 case. Consolidate the functionality
into a function to reduce code duplication.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Merge remote-tracking branch 'upstream/master'