Detect whether the "standard" targets (lz4, glm, and imgui) already exist and use them instead of redefining. Rename "non-standard" targets (those without their own CMakeLists, such as stb) to a ...
cmake .. -G "Visual Studio 16 2019" -A x64 -DCMAKE_INSTALL_PREFIX=D:/MyOsgEarthQt/OpenSceneGraph-3.6.4/install -DCMAKE_TOOLCHAIN_FILE=D:/OSG_SOURCE/vcpkg/scripts ...