gnu: gajim: Honour $GAJIM_PLUGIN_PATH.

This allows Gajim to find plug-ins installed with Guix.
Reported at <http://issues.guix.gnu.org/44730>.

However, at the time of pushing both packaged plug-ins (gajim-omemo and
gajim-openpgp) require an unstable version of Gajim and will not load.

* gnu/packages/messaging.scm (gajim)[arguments]: Replace the
‘add-plugin-dirs’ substitution phase with a...
[source]: ...patch that won't silently break in the future.
* gnu/packages/patches/gajim-honour-GAJIM_PLUGIN_PATH.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.

gnu/local.mk

@@ -990,6 +990,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/freedink-engine-fix-sdl-hints.patch	\
   %D%/packages/patches/freeimage-unbundle.patch		\
   %D%/packages/patches/fuse-overlapping-headers.patch				\
+  %D%/packages/patches/gajim-honour-GAJIM_PLUGIN_PATH.patch	\
   %D%/packages/patches/ganeti-deterministic-manual.patch	\
   %D%/packages/patches/ganeti-disable-version-symlinks.patch	\
   %D%/packages/patches/ganeti-drbd-compat.patch			\

gnu/packages/messaging.scm

@@ -743,7 +743,8 @@ of xmpppy.")
                        (version-major+minor version)
                        "/gajim-" version ".tar.gz"))
        (sha256
-        (base32 "1gfcp3b5nq43xxz5my8vfhfxnnli726j3hzcgwh9fzrzzd9ic3gx"))))
+        (base32 "1gfcp3b5nq43xxz5my8vfhfxnnli726j3hzcgwh9fzrzzd9ic3gx"))
+       (patches (search-patches "gajim-honour-GAJIM_PLUGIN_PATH.patch"))))
     (build-system python-build-system)
     (arguments
      `(#:imported-modules
@@ -756,16 +757,6 @@ of xmpppy.")
         (guix build utils))
        #:phases
        (modify-phases %standard-phases
-         (add-after 'unpack 'add-plugin-dirs
-           (lambda _
-             (substitute* "gajim/common/configpaths.py"
-               (("_paths\\['PLUGINS_USER'\\]\\]")
-                "_paths['PLUGINS_USER']] + \
-([os.getenv('GAJIM_PLUGIN_PATH')] \
-if os.getenv('GAJIM_PLUGIN_PATH') \
-and Path(os.getenv('GAJIM_PLUGIN_PATH')).is_dir() \
-else [])"))
-             #t))
          (replace 'check
            (lambda _
              ;; Tests require a running X server.

gnu/packages/patches/gajim-honour-GAJIM_PLUGIN_PATH.patch

@@ -0,0 +1,11 @@
+--- a/gajim/common/configpaths.py	2020-11-19 12:12:02.004414323 +0100
++++ a/gajim/common/configpaths.py	2020-11-19 15:34:52.211476895 +0100
+@@ -47,7 +47,7 @@
+         return [Path(_paths['PLUGINS_BASE']),
+                 Path('/app/plugins')]
+     return [Path(_paths['PLUGINS_BASE']),
+-            Path(_paths['PLUGINS_USER'])]
++            Path(_paths['PLUGINS_USER'])] + ([Path(os.getenv('GAJIM_PLUGIN_PATH'))] if os.getenv('GAJIM_PLUGIN_PATH') and Path(os.getenv('GAJIM_PLUGIN_PATH')).is_dir() else [])
+ 
+ 
+ def get_paths(type_: PathType) -> Generator[str, None, None]: