Thanks @manuelbl.
There were line feed after the #endif. But I finally found out what was wrong. It was simply the order of the includes.
Including TimeManager before LightManager makes everything work. The only difference in includes between the two is mbed.h, and including mbed.h in LightManager allows me to include TimeManager and LightManager in any order.
I suspect that Arduino.h and/or mbed.h do some weird initialization…
Thanks anyways !