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,
witch'/></form></td></tr>
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<tr><td><span title='2021-12-23 10:54:01 +0100'>2021-12-23</span></td><td><a href='/guix/commit/gnu/tests/reconfigure.scm?id=7682145b9b05a500a3a6509960be22b865ca4154'>tests: reconfigure: Fix it.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2021-09-26 23:20:56 +0200'>2021-09-26</span></td><td><a href='/guix/commit/gnu/tests/reconfigure.scm?id=89b05442933d92f4b21c3294455a498e8333727d'>tests: Reduce boilerplate for users of 'system-test-runner'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-09-25 18:52:05 +0200'>2021-09-25</span></td><td><a href='/guix/commit/gnu/tests/reconfigure.scm?id=1fb75128a5202b406622da6272bc809c46d1f439'>tests: Adjust to SRFI-64 as found in Guile 3.0.7.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-08-29 01:05:26 -0400'>2021-08-29</span></td><td><a href='/guix/commit/gnu/tests/reconfigure.scm?id=2ca982ff41270288913ad6b7d5d9e1cad87b06d9'>gnu: bootloader: Support multiple targets.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>