Use the new cur_instance variable to get the call handle instead of calling
vpi_handle(vpiSysTfCall, 0).
This completes a proper fix for issue #141, to replace the problematic fix
that was reverted in commit 8da8261f.
The Cygwin compiler is a bit picky. This patch adds some casts
to remove compilation warnings. In the past I have had warnings
off because of problems with the STL, but for C directories
like this it makes sense to enable the warnings. It also does
not recognize that an assert(0) or assert(false) ends a routine
so it complains about no return at end of function or variables
not being defined.