As the name of the .tar.gz implies (well, to me, anyway) the sqsh binary for Cygwin available on my site was built with OpenClient 15 (that comes with ASE 15.0.2, free download from
www.sybase.com).
This binary
may work with OCS 12.5 - try to go to the %SYBASE%/OCS-12_5/dll directory and copy (copy - not rename!!) the files libct.dll, libcs.dll and libblk.dll to libsybct.dll, libsybcs.dll and libsybblk.dll respectively.
Otherwise you can try to rebuild sqsh from source - there is documentation in the source package on how to do this.
Michael