Disable deprecation warning for old boost

There are a few version of boost which deprecated global placeholders
but do not yet support std placeholders. Simply suppress warning for
those cases.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

source/third_party/jsonspirit/json_spirit_reader_template.h

@@ -22,6 +22,7 @@
 #   include <functional>
     using namespace std::placeholders;
 #else
+#   define BOOST_BIND_GLOBAL_PLACEHOLDERS 1
 #   include <boost/bind.hpp>
 #endif