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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<div class='path'>path: <a href='/guix/log/?id=dda6b8b99f69ab253f11da166ffc4d81869d6278'>root</a>/<a href='/guix/log/tests?id=dda6b8b99f69ab253f11da166ffc4d81869d6278'>tests</a>/<a href='/guix/log/tests/gremlin.scm?id=dda6b8b99f69ab253f11da166ffc4d81869d6278'>gremlin.scm</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/tests/gremlin.scm?id=dda6b8b99f69ab253f11da166ffc4d81869d6278&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-06-23 20:30:22 -0700'>2021-06-23</span></td><td><a href='/guix/commit/tests/gremlin.scm?id=e4b5a23841bc6a92e2e21dd8b1038addee55eb02'>tests: gremlin: Skip file-needed/recursive if DT_NEEDED is empty.</a><span class='msg-avail'>...</span></td><td>Chris Marusich</td></tr>
<tr><td><span title='2021-04-22 14:04:27 +0200'>2021-04-22</span></td><td><a href='/guix/commit/tests/gremlin.scm?id=1010171280cb34c8dc361644b66fdd1a041d297a'>gremlin: 'elf-dynamic-info' returns a file name rathern than a dynamic entry.</a><span class='msg-avail'>...</span></td><td>Dion Mendel</td></tr>
<tr><td><span title='2020-12-01 21:30:28 +0100'>2020-12-01</span></td><td><a href='/guix/commit/tests/gremlin.scm?id=53fd256e5ba43e516fb9d6eaf085b88fe8bd12b6'>gremlin: Add 'file-needed/recursive'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-19 17:28:46 +0200'>2020-09-19</span></td><td><a href='/guix/commit/tests/gremlin.scm?id=49a1203d67a90a6a7ce4e4537697a6da96ceb213'>gremlin: Add 'set-file-runpath', 'file-runpath', and 'file-needed'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>