[PostGIS] #5655: make check fails when you do not have xmllint installed

[PostGIS]

#5655: make check fails when you do not have xmllint installed
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 Reporter:  strk                      |      Owner:  strk
     Type:  defect                    |     Status:  new
 Priority:  medium                    |  Milestone:  PostGIS 3.4.2
Component:  build                     |    Version:  3.4.x
 Keywords:  documentation, testsuite  |
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 In stable-3.4 branch you can ./configure just fine and even "make" but
 "make check" fails complaining about a missing "xmllint".

 I'd prefer for "make check" to just skip testing documentation when
 documentation cannot be built instead of erroring out.
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/5655>
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