Add web assembly support (emscripten/wasm).

include/boost/config/platform/wasm.hpp

@@ -0,0 +1,17 @@
+//  (C) Copyright John Maddock 2020.
+//  Use, modification and distribution are subject to the
+//  Boost Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+//  See http://www.boost.org for most recent version.
+
+//  WASM specific config options:
+
+#define BOOST_PLATFORM "Wasm"
+
+// boilerplate code:
+#include <boost/config/detail/posix_features.hpp>
+//
+// fenv lacks the C++11 macros:
+//
+#define BOOST_NO_FENV_H