Allow stdatomic.h to be included from mingw prebuilt.

Define __GNUC_PREREQ if it isn't already defined.
This is a bit ugly, but it seems to be easiest to make this header as
context-independent as possible.  Together with the addition of a symbolic
link to this file from the mingw prebuilt, this appears to unblock
commit e9fa6be333e35d9e7ae435aeb32532875b95fe22.

Change-Id: I97e39cda8d8f9aa108aa61c4121da09eb9739062
Assume glibc >= 2.15.

This catches one trivial difference between us and glibc --- the error
returned by pthread_setname_np for an invalid pthread_t.

Change-Id: If4c21e22107c6488333d11184f8005f8669096c2
Don't receive structs containing pointers over sockets.

Fixes x86-64 netd.

Change-Id: Iee5ef802ebbf2e000b2593643de4eec46f296c04
Merge "Allow stdatomic.h to be included from mingw prebuilt."
Merge "Don't receive structs containing pointers over sockets."
Merge "Assume glibc >= 2.15."