use CMake to build external 3rd party modules
setup CI for all platforms to ensure 3rd party works
use modern cmake (target_*), for 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)