compat.h: HPUX does not have MSG_WAITALL flag

So I implemented a solution found at the source
of all knowledge, StackOverflow.

And yes, I am compiling a MongoDB client on HPUX.

Signed-off-by: Viktor Tusa <tusa@balabit.hu>
Merge pull request #20 from talien/f/hpux-msg-waitall

compat.h: HPUX does not have MSG_WAITALL flag