Docset is an increasingly popular format that is used to generate
an index for HTML documentation that can be viewed by specialized
viewers such as Dash (on the Mac). The output can only be generated
on a Mac currently. Enabling the DOCSET parameter will generate a
Makefile in the html output directory, which can be used to generate
the Docset itself.
Enabling the Docset Makefile does not negatively impact other things,
as such it seems safe to just enable it for whoever wants to build
the docset on a mac.
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