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
Updated Assimp defines to be more flexible

This commit is a follow-up to 85b20197671e142044e471df603debd0faf08baf

Why was export.h removed from assimp < 3.0.0?
Invert Assimp version detect logic for greater accuracy
Fix export API detection (for assimp < 2.0.885)

It looks like this API was added in Assimp 2.0.885:
https://github.com/assimp/assimp/commit/ae23c03bd9a0b5f1227dc0042fd98f7206c770a8
Use VERSION_LESS instead of STRLESS

The version comparison routines were added in cmake 2.8.0
Merge pull request #53 from cottsay/hydro-devel

Better feature detection for assimp version