libcap-ng: bump revision to 0.7.3

Also cleaned the package makefile up by removing explicitly naming
the source along with moving the configure of the package down below
the standard package declarations.

Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>