Merge hardware POPCNT detection and use

Tests on Joona luxury iCore7 QUAD show that speed increase
against standrd 64bit routine is between 3% and 4%.

So it seems a good thing to have. Also the user feedback at
startup regarding the compile and the hardware detection can
be an useful debug tool.

No functional change.

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

src/movegen.cpp

@@ -24,6 +24,7 @@
 
 #include <cassert>
 
+#include "bitcount.h"
 #include "movegen.h"
 
 // Simple macro to wrap a very common while loop, no facny, no flexibility,