assimp::assimp-NOTFOUND

[Lecrapouille]

Not a real problem but I got plenty of makefile rules (generated by CMake) depending on the "assimp::assimp-NOTFOUND" file making failed the build. Removing them and adding the -lassimp in the link.txt make pass the build.

Build with Debian 10 AMD64 (I had first difficulty to localize assimp:

CMake Error at threepp/quick/CMakeLists.txt:27 (add_library):
  Target "threeppq" links to target "assimp::assimp" but the target was not
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?
```) so I compiled/installed assimp manually from GitHub repo