PostgreSQL 9.1 backend failure on debbie when compiling with SFCGAL support

[robe]

Sadly 9.1 is a version we said we'd support in PostGIS 2.2.

​http://debbie.postgis.net:8080/job/PostGIS_Regress/2398/consoleFull

 regress_sfcgal .. ok 
 sfcgal/empty .. failed (diff expected obtained: /var/lib/jenkins/workspace/postgis/tmp/2_2_pg9.1w64/test_107_diff)
-----------------------------------------------------------------------------
--- sfcgal/empty_expected	2015-11-30 18:26:40.635232931 +0000
+++ /var/lib/jenkins/workspace/postgis/tmp/2_2_pg9.1w64/test_107_out	2015-11-30 19:25:24.220663072 +0000
@@ -1,3 +1,4 @@
+ERROR:  unrecognized configuration parameter "postgis.backend"
 T1.1|POINT EMPTY
 T1.2|LINESTRING EMPTY
 T1.3|SRID=4326;POLYGON EMPTY
@@ -30,7 +31,7 @@
 ST_Intersection(empty, empty) == empty|t
 ST_Difference(geometry, empty) == geometry|t
 ST_Difference(empty, geometry) == empty|t
-ST_Distance(geometry, empty) == NULL|inf
+ST_Distance(geometry, empty) == NULL|
 ST_DWithin(geometry, empty, tolerance) == FALSE|f
 ST_Within(geometry, empty) == FALSE|f
 ST_Contains(empty, geometry) == FALSE|f
-----------------------------------------------------------------------------
 sfcgal/geography .. failed (diff expected obtained: /var/lib/jenkins/workspace/postgis/tmp/2_2_pg9.1w64/test_108_diff)
-----------------------------------------------------------------------------
--- sfcgal/geography_expected	2015-11-30 18:26:40.923232666 +0000
+++ /var/lib/jenkins/workspace/postgis/tmp/2_2_pg9.1w64/test_108_out	2015-11-30 19:25:24.288663146 +0000

I think it's rare though that people will be using SFCGAL with 9.1. So we have couple options here:

a) Fix the issue so backend switching works for 9.1. Since we don't support 9.1 in 2.3, we don't need to make the fix in 2.3

b) Disable SFCGAL testing in 2.2 (at least for 9.1) and tell people to upgrade if they want to use SFCGAL in 9.1 in 2.2

c) Just stop testing 9.1 on 2.2

[robe]

Okay per pramsey's IRC note - going for plan C. Changed debbie's job to no longer bother testing 9.1 in 2.2

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[14:03] <robe2> pramsey and what about this one - https://trac.osgeo.org/postgis/ticket/3386 which option seems least offensive?
[14:03] <sigq> Title: #3386 (PostgreSQL 9.1 backend failure on debbie when compiling with SFCGAL support) – PostGIS (at trac.osgeo.org)
[14:04] <pramsey> just stop testing
[14:04] <robe2> yeh I was hoping you'd say that - less work for me :)

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