In order to build ASYN on cygwin 1.5.x you need to download
and install the cygwin "Sun RPC" module.  On 1.7.x you need
the "tirpc" module.

You should add one of the following lines to 
base/configure/os/CONFIG_SITE.cygwin-x86.cygwin-x86

For Cygwin 1.7.x:
CYGWIN_RPC_LIB = tirpc

For Cygwin 1.5.x
CYGWIN_RPC_LIB = rpc

