When I did the change as described above
I did get this response ...
$ gcc stringecho.c -O3 -Wall -swc -o stringecho.swc
MS-DOS style path detected: N:/cygwin/alcsdk05/avm2-libc/include
Preferred POSIX equivalent is: /alcsdk05/avm2-libc/include
CYGWIN environment variable option "nodosfilewarning" turns off this warning.
Consult the user's guide for more details about POSIX paths:
WARNING: While resolving call to function 'main' arguments were dropped!
This is cygwin 1.7.
In the end, the new stringecho.swc worked.
EchoTest.as printed echo to the Output console as expected.