__CYGWIN32__ is not defined when building with the 64-bit Cygwin
toolchain. According to the Cygwin FAQ, __CYGWIN__ has been defined
since 1998, so this should still work for users on 32-bit systems.
The old scheme of linking the VPI modules with the vvp exports meant they
did not work when loaded by the compiler. Instead, let each client create
a jump table for the VPI routines and pass that to each VPI module as it
is loaded.