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='2020-05-20 08:30:35 -0400'>2020-05-20</span></td><td><a href='/guix/commit/gnu/bootloader/depthcharge.scm?id=b460ba7992a0b4af2ddb5927dcf062784539ef7b'>bootloader: grub: Allow booting from a Btrfs subvolume.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2019-04-03 18:21:42 +0200'>2019-04-03</span></td><td><a href='/guix/commit/gnu/bootloader/depthcharge.scm?id=96c90474c0cbfe88d1dccb9cb52a0f1b3a1505e7'>gnu: Add configuration for depthcharge bootloader.</a><span class='msg-avail'>...</span></td><td>Timothy Sample</td></tr>