Prepare for crosscompile support

cmake "cleanups" (https://github.com/assimp/assimp/issues/364)

feel free to cherry-pick
https://github.com/arris69/assimp/commit/c6e2d1311591ba20091f0685797648a14909fd64
https://github.com/arris69/assimp/commit/55ce03919ef225348e3cbd040ad54d4d611ca350
https://github.com/arris69/assimp/commit/230a00487d98f5d43e6cefbd6ea1046bd20f8c0b
https://github.com/arris69/assimp/commit/3e7968365f6d8cd510678663690b4fe09d082b39

for travis yes, for humans probably not :-)