Support the PURPLE_PLUGIN_PATH environment variable.  This patch is based on
one from Nixpkgs, but properly handles the case where that variable is unset.

--- pidgin-2.10.11/libpurple/plugin.c.orig	2014-11-23 11:41:25.000000000 -0500
+++ pidgin-2.10.11/libpurple/plugin.c	2015-08-04 14:31:02.257366144 -0400
@@ -1178,8 +1178,19 @@
 void
 purple_plugins_init(void) {
 	void *handle = purple_plugins_get_handle();
+	const gchar *purple_plugin_path;
 
 	purple_plugins_add_search_path(LIBDIR);
+	purple_plugin_path = g_getenv("PURPLE_PLUGIN_PATH");
+	if (purple_plugin_path && *purple_plugin_path) {
+		gchar **paths = g_strsplit(purple_plugin_path, ":", -1);
+		if (paths) {
+			gchar **p;
+			for (p = paths; *p; ++p)
+				if (**p) purple_plugins_add_search_path(*p);
+		}
+		g_strfreev(paths);
+	}
 
 	purple_signal_register(handle, "plugin-load",
 						 purple_marshal_VOID__POINTER,
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<tr><td><span title='2020-09-05 22:30:04 +0300'>2020-09-05</span></td><td><a href='/guix/commit/gnu/packages/wget.scm?id=de3c03a47160dec355d9b19ad5ca210d90c15fd7'>Merge remote-tracking branch 'origin/master' into staging</a></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-09-01 04:44:34 +0200'>2020-09-01</span></td><td><a href='/guix/commit/gnu/packages/wget.scm?id=30714da09a50bf20780e9903104e29c930da855a'>gnu: wgetpaste: Update bpaste URL &amp; regular expressions.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/wget.scm (wgetpaste)[source]: Add patch.
* gnu/packages/patches/wgetpaste-update-bpaste.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-09-01 04:44:34 +0200'>2020-09-01</span></td><td><a href='/guix/commit/gnu/packages/wget.scm?id=7b88396febe35e05bb4f67ed609204c8533314d8'>gnu: wgetpaste: Update to 2.30.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/wget.scm (wgetpaste): Update to 2.30.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-09-01 04:44:34 +0200'>2020-09-01</span></td><td><a href='/guix/commit/gnu/packages/wget.scm?id=1b0755012b38fded6059c10849dc16e3e50ed8b8'>gnu: wgetpaste: Use HTTPS home page.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/wget.scm (wgetpaste)[source, home-page]: Use HTTPS.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-09-01 04:44:34 +0200'>2020-09-01</span></td><td><a href='/guix/commit/gnu/packages/wget.scm?id=57ea4dfca1212687213a2db8cd47addf85e2ea3f'>gnu: wgetpaste: Fix historical confusion.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/wget.scm (wgetpaste)[arguments]: Fix the reversed
substitution and clarify its purpose.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-07-03 10:56:21 +0200'>2020-07-03</span></td><td><a href='/guix/commit/gnu/packages/wget.scm?id=8a6898b60173ff9c70f53a5641335c2e99d51a7e'>gnu: wget: Cross-build fix.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/wget.scm (wget)[inputs]: Move lzip to ...
[native-inputs]: ... here.
</span></span></td><td>Jan (janneke) Nieuwenhuizen</td></tr>
<tr><td><span title='2020-04-22 23:35:54 +0300'>2020-04-22</span></td><td><a href='/guix/commit/gnu/packages/wget.scm?id=2213d00faca72029dc54b559dd6124059c6c0881'>gnu: wget2: Add support for gnu updater.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/wget.scm (wget2)[properties]: New field. Add
ftp-directory.
</span></span></td><td>Efraim Flashner</td></tr>