A similar problems for the above (i. e. non-atomic) branch was addressed in #148, yet the default C++11 branch causes segault with the following stacktrace:
```
(gdb) bt
#0 0x000000000e1c3d28 in boost::container::pmr::memory_resource::allocate (this=0x0, bytes=92, alignment=4)
at boost/container/include/boost/container/pmr/memory_resource.hpp:48
#1 0x000000000e2e9e59 in boost::container::pmr::polymorphic_allocator<float>::allocate (this=0x7fffffffd148, n=23)
at boost/container/include/boost/container/pmr/polymorphic_allocator.hpp:92
#2 0x000000000e2e9cdd in std::__y1::allocator_traits<boost::container::pmr::polymorphic_allocator<float> >::allocate (__a=..., __n=23)
at libcxx/include/__memory/allocator_traits.h:262
#3 0x000000000e2e9b13 in std::__y1::vector<float, boost::container::pmr::polymorphic_allocator<float> >::__vallocate (this=0x7fffffffd130, __n=23)
at libcxx/include/vector:958
#4 0x000000000e33c591 in std::__y1::vector<float, boost::container::pmr::polymorphic_allocator<float> >::vector (this=0x7fffffffd130, __n=23)
at libcxx/include/vector:1106
```
do_allocate() triggered the warning
"code will never be executed [-Wunreachable-code]"
in Clang.
Changed both do_allocate and do_deallocate to keep the existing
similarity.
- 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.
