Hadrian fails to rebuild Evac_thr.c when Evac.c changes

Evac_thr.c is a simple stub #include'ing Evac.c:

#if defined(THREADED_RTS)    
#define PARALLEL_GC    
#include "Evac.c"    
#endif    

However, Evac_thr.thr_o.d makes no mention of this fact:

_build/stage1/rts/build/c/sm/Evac_thr.o: rts/sm/Evac_thr.c    

Consequently, when Evac.c changes Evac_thr.o does not get rebuilt. This may have just cost me a several hours of head-scratching.