Besides mixing some unrelated (and meanwhile obsolete) patches
into the actual patch, there is one fundamental issue with
this patch: it removes the -mno-cygwin option from CFLAGS
for Cygwin builds.  However, -mno-cygwin has been explicitly
added to allow Win32-API-only builds (i.e. MinGW32 builds)
even when building under Cygwin, so no dependency to
cygwin1.dll is required.  As far as I know, this feature is
crucial to the way the WinAVR build environment works.

For that reason, adding a native Cygwin build should be
implemented as a configure option (like --enable-native-cygwin),
defaulting to the current way (i.e. --disable-native-cygwin).
This is an acceptance requirement.

Also, as Cygwin basically offers a Posix-compliant API, I
think it would be better to massage the build system so
native Cygwin builds just go ahead, and use ser_posix.c
directly, rather than duplicating much of its functionality
in ser_win32.c.  This is not a requirement, but a wish that
allows for easier maintenance.

I'd kindly ask to submit a modified patch.