We can always provide a function rather than a value.
This is what we do in most places already and is consistent with the
rest of KDE.
This gets compiled to the same code.
```cpp
explicit QLoggingCategoryMacroHolder(const QLoggingCategory &cat)
{
if (IsOutputEnabled)
init(cat);
}
explicit QLoggingCategoryMacroHolder(QMessageLogger::CategoryFunction catfunc)
{
if (IsOutputEnabled)
init(catfunc());
}
```
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