assimp: new package

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
----
Changes v1 -> v2:
  - use github helper (thanks to Jörg Krause)

Changes v2 -> v3:
  - add c++ dependency (suggested by Thomas Petazzoni)
  - fix linking problem with builtin zlib (linking code
    with/without '-fpic' compiled, see e.g. [2]), workaround by selecting
    buildroot zlib package (failure detected by Thomas Petazzoni [2])

[1] https://cmake.org/pipermail/cmake/2006-March/008482.html
[2] http://lists.busybox.net/pipermail/buildroot/2015-December/146859.html


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