The strict-aliasing rules in the standard constrain user code, not implementation code. Since the POSIX headers and libraries are part of the implementation, there is no actual conflict between the POSIX and the C standard.
In an open-source platform, and in particular in Linux where the C library and compiler are developed by different teams, this makes life difficult for implementors, but that is their concern, not yours. For example, the implementors could:
- refrain from exposing the potential conflict between the standards (that is, disable strict-aliasing optimizations);
- admit that their implementation is not POSIX compliant (and note that, for example, there are no POSIX-certified Linux distributions);
- provide facilities to ensure that the potentially conflicting facilities do not actually conflict. From the point of view of the C standard, this would be an extension.
This last option is how the gcc and glibc teams are working to resolve the sockaddr issue; see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71255
