cmake problem with strigi

[Marc Espie]

I can't seem to configure strigi simply. On OpenBSD, some system headers
are not self-sufficient. So the strigi check
CHECK_INCLUDE_FILE_CXX(sys/socket.h HAVE_SYS_SOCKET_H)
fails, because sys/socket.h needs sys/types.h to work.

A quick glance at the cmake setup does not make it seem like this kind
of issue has been taken into account, namely, CHECK_INCLUDE_FILE_CXX
does not provide for extra headers...

I can work around it, but I would prefer not to, I think... 
So, either I missed some thing, or I need some design advice...