GCC4 warning fixes part4

Hi,

here's the next part. Changelog:

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Silence warnings in datasource.c. The original code gave these
kinds of warnings on GCC4:
warning: ISO C forbids conversion of function pointer to object pointer type
warning: ISO C forbids initialization between function pointer and 'void *'
warning: ISO C forbids assignment between function pointer and 'void *'
The above warnings sound serious but on the other hand, explicitly
casting the address of a function pointer to a (gpointer *) gave this
warning on older GCCs:
warning: dereferencing type-punned pointer will break strict-aliasing rules
Aliasing function pointers to gpointer using unions solves this, using
   union {
     gpointer ptr;
     void (*function)(void);
   }
allows the assignment to the gpointer and calling function() without
warnings.
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Best regards,
Zoltán Böszörményi