[PostGIS] #5687: PostgreSQL 17 regression failures with spgist indexes

[PostGIS]

#5687: PostgreSQL 17 regression failures with spgist indexes
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  Reporter:  robe     |      Owner:  robe
      Type:  defect   |     Status:  reopened
  Priority:  blocker  |  Milestone:  PostGIS 3.5.0
 Component:  postgis  |    Version:  3.4.x
Resolution:           |   Keywords:
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Comment (by robe):

 I'm having trouble building PG17 under mingw and reverted 0a5d454 as it
 caused other issues and is not the culprit since the regress_index and
 regress_nd_index which use the same function are working fine.

 Some observations in the gist_index code, there is no logic to try to find
 location of postgis extension, yet I see this code in spgist. I'm assuming
 that is where it's failing -
 https://git.osgeo.org/gitea/postgis/postgis/src/branch/master/postgis/gserialized_spgist_2d.c#L296

 I see similar in the spgist_3d.

 Not sure why we have a config function for these and it's not needed for
 the gist one.
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/5687#comment:5>
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