Make snmalloc build on Windows with Clang

Fixes a few places where Clang complains about Windows specific code,
and also uses macros supported by Clang on Windows.  A few places
separating platform and compiler specific code, as MSVC and WIN32 were
used interchangably previously.

src/test/setup.h

@@ -25,7 +25,7 @@ void print_stack_trace()
   {
     // SymInitialize failed
     error = GetLastError();
-    printf("SymInitialize returned error : %d\n", error);
+    printf("SymInitialize returned error : %lu\n", error);
     return;
   }