Enable prefetch also for gcc

This fix a compile error under Linux with gcc when
there aren't the intel dev libraries.

Also simplify the previous patch moving TT definition
from search.cpp to tt.cpp so to avoid using passing a
pointer to TT to the current position.

Finally simplify do_move(), now we miss a prefetch in the
rare case of setting an en-passant square but code is
much cleaner and performance penalty is almost zero.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/tt.h

@@ -105,4 +105,6 @@ private:
   uint8_t generation;
 };
 
+extern TranspositionTable TT;
+
 #endif // !defined(TT_H_INCLUDED)