Better feature detection for assimp version

The unified headers were introduced in Assimp 2.0.1150, so checking for Assimp 3.0.0 is not quite the best solution.

See https://github.com/assimp/assimp/commit/6fa251c2f2e7a142bb861227dce0c26362927fbc

Tested on Fedora 19 (Assimp 2.0.1071) and Fedora 20 (Assimp 3.0.1264).

Should probably be tested on Ubuntu before release, but I don't foresee any issues.

Also, a backport to groovy-devel would be nice!