Fix building on Cygwin.

Fix building on Cygwin.

There were two issues: first, feature test macros were only defined
for __linux__, so defintions we needed were not included. Enable the
same feature test macros for __CYGWIN__.

Second, the Cygwin headers do not define TIOCOUTQ, needed to use
ioctl() to get the number of bytes in the output queue. Return
SP_ERR_SUPP on Cygwin for this operation.

This fixes bug #963.