[PostGIS] #6019: Remove generated files in distclean target

[PostGIS]

#6019: Remove generated files in distclean target
----------------------------+---------------------------
 Reporter:  Bas Couwenberg  |      Owner:  strk
     Type:  patch           |     Status:  new
 Priority:  medium          |  Milestone:  PostGIS 3.6.2
Component:  build           |    Version:  3.6.x
 Keywords:                  |
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 The Debian package failed to build twice because `postgis/cunit/Makefile`
 is generated by `configure` but not removed.
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/6019>
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