This is still causing an issue on OSX Yosemite - I don't know why the CMakeLists isn't correctly evaluating if (NOT \${ASSIMP_VERSION} VERSION_LESS "2.0.1150")
As a hack, I just forced on the ASSIMP_UNIFIED_HEADER_NAMES definition and the package built. I did this by moving the add_definitions(-DASSIMP_UNIFIED_HEADER_NAMES) line outside the if statement.