Fix problem with linker when using mingw

I tried using mingw with make build ARCH=x86-64 COMP=mingw  under Ubuntu1204 and gcc 4.6.3 and received this error message: 

thread.o: In function `~Thread':
/home/tobias/Downloads/FishCooking-equal_see_pruning_depth_0/src/thread.cpp:68: undefined reference to `pthread_join'
thread.o: In function `Thread':
/home/tobias/Downloads/FishCooking-equal_see_pruning_depth_0/src/thread.cpp:54: undefined reference to `pthread_create'
collect2: ld returned 1 exit status
make[1]: *** [stockfish] Error 1

Some reading in the internet brought me to this change but I must admit that I do not understand much of this matter.