nixpkgs: move postgresql patches into a common directory

This simply makes it easier to browse the PostgreSQL package directory. More to
come.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

pkgs/servers/sql/postgresql/patches/hardcode-pgxs-path-96.patch

@@ -0,0 +1,14 @@
+diff -Naur postgresql-9.6.1-orig/src/common/config_info.c postgresql-9.6.1/src/common/config_info.c
+--- postgresql-9.6.1-orig/src/common/config_info.c	2016-11-22 21:39:29.231929261 +0100
++++ postgresql-9.6.1/src/common/config_info.c	2016-11-22 23:36:53.685163543 +0100
+@@ -118,7 +118,10 @@
+ 	i++;
+
+ 	configdata[i].name = pstrdup("PGXS");
++	strlcpy(path, "HARDCODED_PGXS_PATH", sizeof(path));
++/* commented out to be able to point to nix $out path
+ 	get_pkglib_path(my_exec_path, path);
++*/
+ 	strlcat(path, "/pgxs/src/makefiles/pgxs.mk", sizeof(path));
+ 	cleanup_path(path);
+ 	configdata[i].setting = pstrdup(path);