Consistent use of anonymous namespace

Objects that are only accessible at file-scope should be put in the anonymous namespace.
This is what the  C++ standard recommends, rather than using static, which is really C-style and results in static linkage.

Stockfish already does this throughout the code. So let's weed out the few exceptions,
because... they have no reason to be exceptional.

No functional change.

Resolves #84

src/benchmark.cpp

@@ -32,7 +32,9 @@
 
 using namespace std;
 
-static const char* Defaults[] = {
+namespace {
+
+const char* Defaults[] = {
   "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1",
   "r3k2r/p1ppqpb1/bn2pnp1/3PN3/1p2P3/2N2Q1p/PPPBBPPP/R3K2R w KQkq - 0 10",
   "8/2p5/3p4/KP5r/1R3p1k/8/4P1P1/8 w - - 0 11",
@@ -65,6 +67,7 @@ static const char* Defaults[] = {
   "8/3p3B/5p2/5P2/p7/PP5b/k7/6K1 w - - 0 1"
 };
 
+} // namespace
 
 /// benchmark() runs a simple benchmark by letting Stockfish analyze a set
 /// of positions for a given limit each. There are five parameters: the