Preserve gitignore files when cleaning native builds
Change `CModule` shared makefile so that it adapts according to whether its including makefile's component has a [./]gitignore file in one or more of its subdirectories. Git will consider the deletion of such files as significant, and that is usually not the developer's intention. This requires a fully-functional version of amu's `$(wildcard)` function, which was only introduced at amu 5.33, so use RiscOS/Env!5 to adapt according to whether a new enough version is installed.
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