Add assimp as external project, build as static lib, only include desired importers (keep lib size down).

add_assimp.cmake

include(ExternalProject)

ExternalProject_Add(assimp
    PREFIX assimp
    GIT_REPOSITORY https://github.com/assimp/assimp.git
    GIT_TAG master
    CMAKE_ARGS  -DCMAKE_INSTALL_PREFIX=${EXTERNAL_INSTALL_LOCATION}/assimp
        -DBUILD_SHARED_LIBS=OFF
        -DASSIMP_BUILD_TESTS=OFF
        # -DOpenGL_GL_PREFERENCE=GLVND # on certain systems
        -DASSIMP_BUILD_ASSIMP_TOOLS=OFF
        -DASSIMP_BUILD_ZLIB=ON
        -DASSIMP_NO_EXPORT=ON
        -DASSIMP_BUILD_ALL_IMPORTERS_BY_DEFAULT=OFF
        -DASSIMP_BUILD_glTF_IMPORTER=TRUE
        -DASSIMP_BUILD_glTF2_IMPORTER=TRUE
        -DASSIMP_BUILD_Ogre_IMPORTER=TRUE # Contains common Animation namespace