Fix CFCUtil_warn to not use fprintf

Under ActivePerl/MSVC, the call to fprintf caused a crash. This is
probably due to a combination of

- Perl redefining fprintf and/or stderr
- Different MSVCRT versions used to compile Perl and Clownfish

Switching to Perl's warnings API fixes the crash.

Also use the simpler vcroak for CFCUtil_die.
Always compile as C++ with MSVC

Although the generated XS is C89-compliant now, it must be compiled in
C++ mode like the rest of the code due to a mismatch between the sizes
of the C++ bool type and the emulated bool type.