2.1.9--2.5.0dev upgrade fails with PostgreSQL 9.5.10

[strk]

From a test run on Debbie (to be enabled as the new Dronie test):

[build:L2832:379s] PostgreSQL 9.5.10 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4, 64-bit
[build:L2833:379s]   Postgis 2.5.0dev - r - 2018-01-10 20:31:27
[build:L2834:379s]   scripts 2.5.0dev r0
[build:L2835:379s]   raster scripts 2.5.0dev r0
[build:L2836:379s]   GEOS: 3.4.2-CAPI-1.8.2 r3921
[build:L2837:379s]   PROJ: Rel. 4.8.0, 6 March 2012
[build:L2838:379s]   GDAL: GDAL 1.10.1, released 2013/08/26
...
[build:L2938:414s]  knn_recheck .. failed (diff expected obtained: /tmp/pgis_reg/test_97_diff)
[build:L2939:414s] -----------------------------------------------------------------------------
[build:L2940:414s] --- knn_recheck_expected     2018-01-10 06:54:48.998742984 +0000
[build:L2941:414s] +++ /tmp/pgis_reg/test_97_out        2018-01-10 20:38:06.218378800 +0000
[build:L2942:414s] @@ -50,18 +50,9 @@
[build:L2943:414s]  #2g|30512|25313.2118|25313.2118
[build:L2944:414s]  #3g|1000|t
[build:L2945:414s]  #3g|500000|t
[build:L2946:414s] -#1g|500000|0.0000|0.0000
[build:L2947:414s] -#1g|600003|69974.6935|69974.6935
[build:L2948:414s] -#1g|2614|70976.1794|70976.1794
[build:L2949:414s] -#1g|2615|75048.8551|75048.8551
[build:L2950:414s] -#1g|2795|86965.9298|86965.9298
[build:L2951:414s] -#2g|31244|4189.9122|4189.9122
[build:L2952:414s] -#2g|31061|6756.6424|6756.6424
[build:L2953:414s] -#2g|30878|15161.4706|15161.4706
[build:L2954:414s] -#2g|30695|21264.3654|21264.3654
[build:L2955:414s] -#2g|30512|25313.2118|25313.2118
[build:L2956:414s] -#3g|1000|t
[build:L2957:414s] -#3g|500000|t
[build:L2958:414s] +ERROR:  missing support function 8 for attribute 1 of index "idx_knn_recheck_geog_gist"
[build:L2959:414s] +ERROR:  missing support function 8 for attribute 1 of index "idx_knn_recheck_geog_gist"
[build:L2960:414s] +ERROR:  missing support function 8 for attribute 1 of index "idx_knn_recheck_geog_gist"
[build:L2961:414s]  #1nd-3|290|260.6797|260.6797
[build:L2962:414s]  #1nd-3|287|264.3000|264.3000
[build:L2963:414s]  #1nd-3|579|265.4356|265.4356
[build:L2964:414s] -----------------------------------------------------------------------------
...
[build:L2991:418s] FAIL: upgrade 2.1.9--2.5.0dev

Note that subsequent upgrade paths succeed, for example:

[build:L3311:507s] PASS: upgrade 2.2.6--2.5.0dev

I hadn't tested that upgrade to 2.2, 2.3 or 2.4

[strk]

In 16348:

Set availability of support function for KNN index

Fixes upgrade from 2.1 to 2.2+
References #3978 in trunk (2.5.0+)
Should be backported up to 2.2 branch

[strk]

In 16356:

Set availability of support function for KNN index

Fixes upgrade from 2.1 to 2.2+
References #3978 in 2.2 branch (2.2.7+)

[strk]

In 16357:

Set availability of support function for KNN index

Fixes upgrade from 2.1 to 2.2+
References #3978 in 2.3 branch (2.3.7)

[strk]

In 16358:

Set availability of support function for KNN index

Fixes upgrade from 2.1 to 2.2+
Closes #3978 in 2.4 branch (2.4.4)