Allow the configuration file to be specified via an environment variable.

--- geoclue-2.1.10/src/gclue-config.c	2015-04-07 09:50:07.721074380 +0200
+++ geoclue-2.1.10/src/gclue-config.c	2015-04-07 10:27:26.613171960 +0200
@@ -235,6 +235,11 @@
 gclue_config_init (GClueConfig *config)
 {
         GError *error = NULL;
+	const char *config_file_path;
+
+	config_file_path = g_getenv ("GEOCLUE_CONFIG_FILE");
+	if (config_file_path == NULL)
+	  config_file_path = CONFIG_FILE_PATH;
 
         config->priv =
                 G_TYPE_INSTANCE_GET_PRIVATE (config,
@@ -242,7 +247,7 @@
                                             GClueConfigPrivate);
         config->priv->key_file = g_key_file_new ();
         g_key_file_load_from_file (config->priv->key_file,
-                                   CONFIG_FILE_PATH,
+                                   config_file_path,
                                    0,
                                    &error);
         if (error != NULL) {
table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=a5fe52001042970d9b7d063d5a218484ab7fa7c5'>refs</a><a class='active' href='/guix/log/THANKS'>log</a><a href='/guix/tree/THANKS?id=a5fe52001042970d9b7d063d5a218484ab7fa7c5'>tree</a><a href='/guix/commit/THANKS?id=a5fe52001042970d9b7d063d5a218484ab7fa7c5'>commit</a><a href='/guix/diff/THANKS?id=a5fe52001042970d9b7d063d5a218484ab7fa7c5'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/THANKS'>
<input type='hidden' name='id' value='a5fe52001042970d9b7d063d5a218484ab7fa7c5'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=a5fe52001042970d9b7d063d5a218484ab7fa7c5'>root</a>/<a href='/guix/log/THANKS?id=a5fe52001042970d9b7d063d5a218484ab7fa7c5'>THANKS</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/THANKS?id=a5fe52001042970d9b7d063d5a218484ab7fa7c5&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>