Have constant link order
to make builds reproducible.
See https://reproducible-builds.org/ for why this is good.
Without this patch g++ would order functions in libboost_container.so.1.64.0
depending on random order of files in the build system's filesystem.
- 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.
