GitLab

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.