Fix issue #557 (workaround?)
Fix issue #557 (workaround?)

If we build an internal plugin `SQUEAK_BUILTIN_PLUGIN`, then we don't need to `EXPORT` the error/warning functions.
There are still conflicting definitions in sqVirtualMachine.h which totally ignore IMPORT/EXPORT but these only generate warnings, not errors.

timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm