Add Polymorphic Memory Resource utilities and rework the separately compiled library:

- Dlmalloc's based C function are boost_cont_xxx no longer exported, but wrapped into C++ linkage dlmalloc_xxx functions to effectively reuse Boost's dynamic library and autolink machinery instead of rewriting machinery to compile the C source file.
- Refactored scoped_allocator_adaptor's construct logic as it was shared with polymorphic allocator's one. Moved common logic to detail/dispatch_uses_allocator.hpp. Refactored also scoped_allocator_adaptor test utilities to be reused with polymorphic_allocator tests.

include/boost/container/pmr/string.hpp

@@ -0,0 +1,48 @@
+//////////////////////////////////////////////////////////////////////////////
+//
+// (C) Copyright Ion Gaztanaga 2015-2015. Distributed under 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/libs/container for documentation.
+//
+//////////////////////////////////////////////////////////////////////////////
+
+#ifndef BOOST_CONTAINER_PMR_STRING_HPP
+#define BOOST_CONTAINER_PMR_STRING_HPP
+
+#if defined (_MSC_VER)
+#  pragma once 
+#endif
+
+#include <boost/container/string.hpp>
+#include <boost/container/pmr/polymorphic_allocator.hpp>
+
+namespace boost {
+namespace container {
+namespace pmr {
+
+#if !defined(BOOST_NO_CXX11_TEMPLATE_ALIASES)
+
+template <class CharT, class Traits = std::char_traits<CharT>  >
+using basic_string =
+   boost::container::basic_string<CharT, Traits, polymorphic_allocator<CharT> >;
+
+#endif
+
+template <class CharT, class Traits = std::char_traits<CharT> >
+struct basic_string_of
+{
+   typedef boost::container::basic_string
+      <CharT, Traits, polymorphic_allocator<CharT> > type;
+};
+
+typedef basic_string_of<char>::type    string;
+
+typedef basic_string_of<wchar_t>::type wstring;
+
+}  //namespace pmr {
+}  //namespace container {
+}  //namespace boost {
+
+#endif   //BOOST_CONTAINER_PMR_STRING_HPP