fix GCC Bug 57350: std::align missing

[templateU]

it will cause compilation failure if use g++<5.0.
The detailed information of the bug can be found: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57350
is that possible to use macro to determine that when the compiler is gcc and the version is less than 5.0, use the source code （g++>=5.0) directly in the call to std::align to obtain asio library support for compilers before gcc 5.0?

[hdu-sdlzx]

This is the only error generated by gcc-4.x.
operator "" in buffer.hpp introduces another compatibility issue:

09:57:59 /home/api/artifact/gcc-4.8.5/boost-1.89.0/include/boost/asio/buffer.hpp:2872:21: error: missing space between ‘""’ and suffix identifier
09:57:59 inline const_buffer operator ""_buf()
09:57:59 ^

The code is introduced in d48d95d since boost-1.84

[hdu-sdlzx]

There was a check about std::align in boost-1.83:
https://github.com/boostorg/asio/blame/boost-1.83.0/include/boost/asio/detail/memory.hpp

inline void* align(std::size_t alignment,
    std::size_t size, void*& ptr, std::size_t& space)
{
#if defined(BOOST_ASIO_HAS_STD_ALIGN)
  return std::align(alignment, size, ptr, space);
#else // defined(BOOST_ASIO_HAS_STD_ALIGN)
	const uintptr_t intptr = reinterpret_cast<uintptr_t>(ptr);
	const uintptr_t aligned = (intptr - 1u + alignment) & -alignment;
	const std::size_t padding = aligned - intptr;
	if (size + padding > space)
    return 0;
	space -= padding;
	ptr = reinterpret_cast<void*>(aligned);
  return ptr;
#endif // defined(BOOST_ASIO_HAS_STD_ALIGN)
}

which is dropped in commit 5c19f29 ("Require C++11 as the minimum c++ standard.")

[hdu-sdlzx]

Since boost.asio is NOT gtested with gcc-4.x (The minimal version in test summary is gcc-6, https://regression.boost.org/develop/developer/summary.html), I suggest we fix the doc and officially stop support for gcc-4.x.
@chriskohlhoff

[hdu-sdlzx]

I've made a PR to the maintainer's repo:
chriskohlhoff/asio#1694

[templateU]

Thanks for your work^^ @hdu-sdlzx