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='2021-05-31 10:45:54 +0200'>2021-05-31</span></td><td><a href='/guix/commit/gnu/packages/pep.scm?id=c85edf2ca47a10634faba15848cca7b4558a82ab'>gnu: java-pep-adapter 2.1.23.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pep.scm (java-pep-adapter): Update to 2.1.23.
  [source]: Switch to new git repo.
  [arguments]&lt;make-flags&gt;: Enable release-build, use new target names.
  [arguments]&lt;phases&gt;{fix-includes}: Remove phase.
  {pin-shared-lib-path}: Update file paths.
  {build-codegen}: New phase.
  {install}: Use "(install)".
  [native-inputs]: Add graphviz.
  [inputs]: Fix input util-linux.
</span></span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2021-05-31 10:45:53 +0200'>2021-05-31</span></td><td><a href='/guix/commit/gnu/packages/pep.scm?id=140087faa50414d5c03f5484b6622170a23a9bcd'>gnu: python-pep-adapter: Update to 2.1.3.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pep.scm (python-pep-adapter): Update to 2.1.3.
  [source]: Switch to new git repo.
  [arguments]&lt;phases&gt;{remove-wheel-requirement}: New phase.
  [native-inputs]: Add python-pytest and python-pytest-forked.
  [inputs]: Remove asn1c, add python-setuptools-scm. util-linux.
</span></span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2021-05-31 10:45:53 +0200'>2021-05-31</span></td><td><a href='/guix/commit/gnu/packages/pep.scm?id=c178f859b5f549da5d064e628a5906d505dd75ef'>gnu: libpEpAdapter: Update to 2.1.21.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pep.scm (libpEpAdapter): Update to 2.1.21.
  [source]: Swicht to new git repo.
  [arguments]&lt;make-flags&gt;: Enable release-build.
</span></span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2021-05-31 10:45:53 +0200'>2021-05-31</span></td><td><a href='/guix/commit/gnu/packages/pep.scm?id=85d80283b6d14b19dacdd687590eba494da4864f'>gnu: pep-engine: Update to 2.1.34.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pep.scm (fdik-libetpan): Update to current development
  version. [source]: Switch to pep.foundation.
  (pep-engine): Update to 2.1.34. [source]: Switch to new git repo.
  [arguments]&lt;make-flags&gt;: Enable release-build.
  [inputs]: Use normal sequoia.
* gnu/packages/sequoia.scm (sequoia4pEp): Remove variable.
</span></span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2021-05-31 10:45:53 +0200'>2021-05-31</span></td><td><a href='/guix/commit/gnu/packages/pep.scm?id=c7f023723c1de3f82a3abfa47b8366fccbd3540a'>gnu: yml2: Update to 2.7.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/pep.scm (yml2): Update to 2.7.0.
  [source]: Switch to new git repo.
</span></span></td><td>Hartmut Goebel</td></tr>
<tr><td><span title='2021-03-22 09:54:19 +0200'>2021-03-22</span></td><td><a href='/guix/commit/gnu/packages/pep.scm?id=4d0fe40f65e096a4d07f4cc8c95139a4dda96c53'>gnu: sequoia4pEp: Move to sequoia.scm.</a><span class='msg-avail'>...<span class='msg-tooltip'>Don't have cross module package inheritance.

* gnu/packages/pep.scm (sequoia4pEp): Move to ...
* gnu/packages/sequoia.scm (sequoia4pEp): ... here. Hide package.
</span></span></td><td>Efraim Flashner</td></tr>