/usr/share/postgresql/9.6/extension/postgis.control is in postgresql-9.6-postgis-2.3-scripts 2.3.1+dfsg-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | # postgis extension
comment = 'PostGIS geometry, geography, and raster spatial types and functions'
default_version = '2.3.1'
module_pathname = '$libdir/postgis-2.3'
relocatable = false
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