BUG: use c compiler for link of c programs, and use temp file nmake syntax for linking c and c++ programs

ENH: Changed check for which version of instantiators to generate to look for definition of VTK_USE_INSTANTIATOR_NEW instead of an extra argument to VTK_MAKE_INSTANTIATOR.

BUG: Added backward-compatability.  The old instantiator style will be used unless the argument USE_INSTANTIATOR_NEW is given to tell the command to make use of the instantiator new functions exported from each class's implementation file.

ENH: Improved generated instantiator to use extern declarations to wrappers around the New() methods.  This avoids the need to include each class's header in an instantiator source.  The instantiator class implementations can now fit in a single source file that compiles quickly.

FIX: fix commit messup (this change was committed to the branch instead of the main tree, thus was wiped-out later)

- WINDOWS does not exist, use WIN32 (or defineWINDOWS if cygwin can not be used inUnix mode?),
- fix an un-closed IF,
- use same prefix for vars (and make it advanced)