meson: add docs, docs_pdf options
Detect and report if the tools necessary to build documentation are available
during configure. This is represented as two new options 'docs' and
'docs_pdf', both defaulting to 'auto'.
This should also fix a meson error about the installdocs target, when none of
the doc tools are found.
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/
20230325201414.sh7c6xlut2fpunnv@awork3.anarazel.de
Discussion: https://postgr.es/m/ZB8331v5IhUA/pNu@telsasoft.com