removed construction of MAKEDEP_FLAGS

this is automagically handled by autotools
check whether we have assimp2 or assimp3

and have two automake conditionals HAVE_ASSIMP2 and HAVE_ASSIMP3
only build modelASSIMP2 if we have libassimp2

and not libassimp3!

Closes https://github.com/umlaeute/Gem/issues/77