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='2018-03-23 04:40:08 -0400'>2018-03-23</span></td><td><a href='/guix/commit/gnu/packages/apr.scm?id=3d77ecfed4aab76fbd1a4b6718e44f42f482a70b'>gnu: apr-util: Use invoke.</a><span class='msg-avail'>...</span></td><td>Mark H Weaver</td></tr>
<tr><td><span title='2017-10-30 21:46:18 -0400'>2017-10-30</span></td><td><a href='/guix/commit/gnu/packages/apr.scm?id=b275cd22d47a42098f465ca30aee24fdc6e23826'>gnu: apr-util: Update to 1.6.1 [fixes CVE-2017-12618].</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2017-10-30 21:46:18 -0400'>2017-10-30</span></td><td><a href='/guix/commit/gnu/packages/apr.scm?id=b614c480178dd751c8bc5713b2052ecda29918d1'>gnu: apr: Update to 1.6.3 [fixes CVE-2017-12613].</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2017-09-20 20:58:04 -0400'>2017-09-20</span></td><td><a href='/guix/commit/gnu/packages/apr.scm?id=dc1d3cdef70f0e3c047c229c2a0e561d1f47bde8'>gnu: Use 'modify-phases' syntax.</a><span class='msg-avail'>...</span></td><td>Kei Kebreau</td></tr>
<tr><td><span title='2016-04-14 10:38:21 +0300'>2016-04-14</span></td><td><a href='/guix/commit/gnu/packages/apr.scm?id=fc1adab1487e5ea3c5f6341263df14414ba4e784'>gnu: packages: Use 'search-patches' everywhere.</a><span class='msg-avail'>...</span></td><td>Alex Kost</td></tr>