Lookup termios symbols in "cygwin1.dll" if %host-system is cygwin

On cygwin, the termios function symbols are located in cygwin1.dll.
This should detect the system-type at compile-time and expand into
the right ‘dynamic-link’ call.

  On cygwin:       (dynamic-link "cygwin1.dll")
  Everywhere else: (dynamic-link)

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
Merge branch 'cygwin'