the plugins get installed to $LOCALBASE/lib/olsrd-plugins, so load them from there.
Index: src/plugin_loader.c
--- src/plugin_loader.c.orig
+++ src/plugin_loader.c
@@ -91,20 +91,14 @@ olsr_load_plugins(void)
 static int
 olsr_load_dl(char *libname, struct plugin_param *params)
 {
-#if defined TESTLIB_PATH && TESTLIB_PATH
-  char path[256] = "/usr/testlib/";
-#endif /* defined TESTLIB_PATH && TESTLIB_PATH */
+  char path[256] = "${LOCALBASE}/lib/olsrd-plugins/";
   struct olsr_plugin *plugin = olsr_malloc(sizeof(struct olsr_plugin), "Plugin entry");
   int rv;
 
   OLSR_PRINTF(0, "---------- LOADING LIBRARY %s ----------\n", libname);
 
-#if defined TESTLIB_PATH && TESTLIB_PATH
-  strcat(path, libname);
+  strlcat(path, libname, sizeof(path));
   plugin->dlhandle = dlopen(path, RTLD_NOW);
-#else /* defined TESTLIB_PATH && TESTLIB_PATH */
-  plugin->dlhandle = dlopen(libname, RTLD_NOW);
-#endif /* defined TESTLIB_PATH && TESTLIB_PATH */
   if (plugin->dlhandle == NULL) {
     const int save_errno = errno;
     OLSR_PRINTF(0, "DL loading failed: \"%s\"!\n", dlerror());