best cross-platform method to get aligned memory

As long as you’re ok with having to call a special function to do the freeing, your approach is okay. I would do your #ifdefs the other way around though: start with the standards-specified options and fall back to platform-specific ones. For example

1. If __STDC_VERSION__ >= 201112L use aligned_alloc.
2. If _POSIX_VERSION >= 200112L use posix_memalign.
3. If _MSC_VER is defined, use the Windows stuff.
4. …
5. If all else fails, just use malloc/free and disable SSE/AVX code.

The problem is harder if you want to be able to pass the allocated pointer to free; that’s valid on all the standard interfaces, but not on Windows and not necessarily with the legacy memalign function some unix-like systems have.