Update: Use C11's aligned_alloc() by default, but keep posix_memalign() via macro...

Update: Use C11's aligned_alloc() by default, but keep posix_memalign() via macro _f_memory_USE_posix_memalign_.

The C11 standard introduced aligned_alloc() making it better practice than posix_memalign().
In case the compiler being used doesn't have aligned_alloc() or the user compiling just wants to posix_memalign() this behavior is preserved via _f_memory_USE_posix_memalign_ macro.

I didn't actually test this beyond confirming that it compiles.
I'm flying blind here.