Hide global visibility when not needed

Also move PieceValue definition in psqt.cpp,
where it is initialized.

Fix a warning in popcount16() with Intel compiler

No functional change.

src/bitboard.cpp

@@ -78,7 +78,7 @@ namespace {
 
   // popcount16() counts the non-zero bits using SWAR-Popcount algorithm
 
-  uint8_t popcount16(uint16_t u) {
+  unsigned popcount16(unsigned u) {
     u -= (u >> 1) & 0x5555U;
     u = ((u >> 2) & 0x3333U) + (u & 0x3333U);
     u = ((u >> 4) + u) & 0x0F0FU;
@@ -152,7 +152,7 @@ const std::string Bitboards::pretty(Bitboard b) {
 void Bitboards::init() {
 
   for (unsigned i = 0; i < (1 << 16); ++i)
-      PopCnt16[i] = popcount16(i);
+      PopCnt16[i] = (uint8_t) popcount16(i);
 
   for (Square s = SQ_A1; s <= SQ_H8; ++s)
   {