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.

bench/bench_alloc_shrink_to_fit.cpp

@@ -126,7 +126,7 @@ void vector_test_template(unsigned int num_iterations, unsigned int num_elements
                   << float(nseconds)/(num_iterations*num_elements)
                   << std::endl << std::endl;
    }
-   boost_cont_trim(0);
+   bc::dlmalloc_trim(0);
 }
 
 int main(int argc, const char *argv[])