Source code for all external dependecies and for all platforms

Use modern cmake (target_*)

Example:

# create library
add_library(Box2D file.cpp)
target_compile_features(Box2D PUBLIC cxx_std_11)
target_include_directories(Box2D PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})

# use library
target_link_libraries(game2 Box2D)

setup CI for all platforms