package/libplist: fix python support

This patch:
- fixes the python3 support detection in the libplist CMake code;
- correctly sets the dependencies and configure options in libplist.mk.

Fixes:
  http://autobuild.buildroot.net/results/efb/efba806b9a40a5e9c6bca7e591747ca2102fa1d5/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>