Problems with SHELL=cmd and mingw32-make

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If there are too many objects, the clean command will fail.

Discovered when building this project:

https://github.com/SpartanJ/eepp

This generated code is problematic:

	$(SILENT) if exist $(subst /,\\,$(TARGET)) del $(subst /,\\,$(TARGET))
	$(SILENT) if exist $(subst /,\\,$(GENERATED)) del /s /q $(subst /,\\,$(GENERATED))
	$(SILENT) if exist $(subst /,\\,$(OBJDIR)) rmdir /s /q $(subst /,\\,$(OBJDIR))

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To be clear, it's trying to execute the object files (.o extension) instead of deleting them. This is extremely hard to debug since there is no --verbose switch or something like that.