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,
ass='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=45ba2caedbc100600c99a27fa448ccceec57ea3b'>refs</a><a class='active' href='/guix/log/etc/git'>log</a><a href='/guix/tree/etc/git?id=45ba2caedbc100600c99a27fa448ccceec57ea3b'>tree</a><a href='/guix/commit/etc/git?id=45ba2caedbc100600c99a27fa448ccceec57ea3b'>commit</a><a href='/guix/diff/etc/git?id=45ba2caedbc100600c99a27fa448ccceec57ea3b'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/git'>
<input type='hidden' name='id' value='45ba2caedbc100600c99a27fa448ccceec57ea3b'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=45ba2caedbc100600c99a27fa448ccceec57ea3b'>root</a>/<a href='/guix/log/etc?id=45ba2caedbc100600c99a27fa448ccceec57ea3b'>etc</a>/<a href='/guix/log/etc/git?id=45ba2caedbc100600c99a27fa448ccceec57ea3b'>git</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/git?id=45ba2caedbc100600c99a27fa448ccceec57ea3b&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-02-08 03:42:08 +0100'>2017-02-08</span></td><td><a href='/guix/commit/etc/git?id=f0d0c5bb188455e0b82ee3089ba103ef71877c87'>etc: The pre-push hook says which commits failed the signature check.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2017-01-04 16:27:20 -0500'>2017-01-04</span></td><td><a href='/guix/commit/etc/git?id=69355e1283d300a663fb639ec426f10f3feb404b'>doc: Add a Git hook that verifies signatures before pushing.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>