Looks like phobos doesn't define it, but druntime does, so you can cast the value to SocketFlags:
// on posix
import core.sys.posix.sys.socket;
receive(buffer, cast(SocketFlags) MSG_WAITALL);
// on windows
receive(buffer, cast(SocketFlags) 0x08 /* MSG_WAITALL, number found on msdn */);
this is a bit hackish, but if you look at the source, you can see the flags is just casted back to int when passed to the low level recv
anyway, so it should work without a problem.