Unsigned negation is a well-defined operation in C and C++ and it is commonly used when doing bit manipulation of unsigned integers, because it builds a high-pass bitmask.
For instance, if `X` is known to be a power of two, `-X` is precisely the mask needed to align a value down to a multiple of `X` by means of a bit-wise `&` operation.
However, it can occasionally be flagging a bug, when the original intent was mathematical negation. For this reason, an explicit fix is preferred to suppressing this warning.
This change fixes all the instances of unsigned negation of the form `-X` to `0 - X`.
It also includes a drive-by spot-fix for the macro `left_bits(x)` whose parameter was missing the usual parenthesis. This was benign in all the uses of the macro.
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.
