#12 autoconf fails for cygwin build

[Chris Marshall]

I just tried building libvisca out of the box on
cygwin 1.7.9 but the compile failed because
FIONREAD is not available on cygwin but
TIOCINQ is. Making the substitution in the
libvisca_posix.c file allowed the build to complete.

$ diff libvisca_posix.c~ libvisca_posix.c
102c102
< ioctl(iface->port_fd, FIONREAD, &(iface->bytes));
---
> ioctl(iface->port_fd, TIOCINQ, &(iface->bytes));
105c105
< ioctl(iface->port_fd, FIONREAD, &(iface->bytes));
---
> ioctl(iface->port_fd, TIOCINQ, &(iface->bytes));
183c183
< ioctl(iface->port_fd, FIONREAD, &bytes);
---
> ioctl(iface->port_fd, TIOCINQ, &bytes);