#84 Build message still refers to "ENABLE_BOOST_WORKAROUND"

[Davide Pasca]

The root CMakeFile.txt has a message that still suggests using:

-DENABLE_BOOST_WORKAROUND=ON

..however, the relevant definition was changed to:

-DASSIMP_ENABLE_BOOST_WORKAROUND

[Kim Kulling]

see: https://github.com/assimp/assimp/issues/126