Fix CFCUtil_warn to not use fprintf
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.

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