What happens now if you rename an enum inside a header:
• glib-mkenums generates the header file
• a comparison is made with the previous version of the heade is made
and no difference is found as you don't remove or create enums
• the compilation of the generated mkenums c file fails because it has
not been regenerated with the new, renamed, enum.
That's why the generated clutter-enum-types.c needs to depend on the
headers too.
Of course such scenario should not happen in stable releases as enums
are part of the API, but renaming enums happens in the development cycle
and create compilation errors (very annoying when doing git bissects for
instance).
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