Can't have debug build coexist with release build

[chenlijun99]

I first build the release version of INET with make MODE=release, then I build the debug version with make MODE=debug. Afterwards the release version must be rebuilt, i.e. if I execute make MODE=release it recompiles everything. I think it is because every build mode re-creates a new header inet/opp_defines.h.

Is there a solution to this limitation?