doc: Flesh out extension docs for the "prefix" make variable

The variable is a bit magical in how it requires "postgresql" or
"pgsql" to be part of the path, and files end up in its "share" and
"lib" subdirectories.  So mention all that and show an example of
setting "extension_control_path" and "dynamic_library_path" to use
those locations.

Author: David E. Wheeler 
Reviewed-by: Matheus Alcantara 
Reviewed-by: Christoph Berg 
Discussion: https://www.postgresql.org/message-id/6B5BF07B-8A21-48E3-858C-1DC22F3A28B4@justatheory.com

diff --git a/doc/src/sgml/extend.sgml b/doc/src/sgml/extend.sgml
index 64f8e133caeb3a39e711794a085be2571bc71574..4915c1985e0b73957d31596ed5f55ab18d4409ef 100644 (file)
--- a/doc/src/sgml/extend.sgml
+++ b/doc/src/sgml/extend.sgml
@@ -1809,10 +1809,36 @@ include $(PGXS)
     setting PG_CONFIG to point to its
     pg_config program, either within the makefile
     or on the make command line.
-    You can also select a separate installation directory for your extension
-    by setting the make variable prefix
-    on the make command line.  (But this will then require
-    additional setup to get the server to find the extension there.)
+   
+
+   
+    You can select a separate directory prefix in which to install your
+    extension's files, by setting the make variable
+    prefix when executing make install
+    like so:
+
+make install prefix=/usr/local/postgresql
+
+    This will install the extension control and SQL files into
+    /usr/local/postgresql/share and the shared modules into
+    /usr/local/postgresql/lib.  If the prefix does not
+    include the strings postgres or
+    pgsql, such as
+
+make install prefix=/usr/local/extras
+
+    then postgresql will be appended to the directory
+    names, installing the control and SQL files into
+    /usr/local/extras/share/postgresql/extension and the
+    shared modules into /usr/local/extras/lib/postgresql.
+    Either way, you'll need to set 
+    linkend="guc-extension-control-path"/> and 
+    linkend="guc-dynamic-library-path"/> to enable the
+    PostgreSQL server to find the files:
+
+extension_control_path = '/usr/local/extras/share/postgresql/extension:$system'
+dynamic_library_path = '/usr/local/extras/lib/postgresql:$libdir'
+