libguile.h indirectly includes libguile/iselect.h via libguile/threads.h and libguile/deprecated.h. iselect.h in turn includes <sys/select.h> that is not provided by MinGW.
<sys/select.h> is included in iselect.h to provide a declaration of fd_set; in MinGW that's declared in winsock2.h.
