multimedia/assimp: unbundle minizip

The port prefers system minizip if available but the build fails due
to an inconsitency between find_package() and pkg_check_modules()
about whether _LIBRARIES should contain absolute paths.

/usr/bin/ld: cannot find -lminizip
c++: error: linker command failed with exit code 1 (use -v to see invocation)

https://gitlab.kitware.com/cmake/cmake/issues/15804

PR: 220889
Reported by: vvd@unislabs.com
Submitted by: yuri@rawbw.com (maintainer, based on)