vnstat: bump to version 1.15

Also:

  - Remove no longer needed patch (merged upstream).
  - Adapt vnstat.mk to the new autotools-based build system.
  - Include patch to improve gd library usability check + add
    configure option to explicitly disable that check.

Signed-off-by: Rodrigo Rebello <rprebello@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>