Make check for writable apps dir configurable

config/config.sample.php

 "log_type" => "",
 "logfile" => "",
 "loglevel" => "",
+/* Set this to false to disable the check for writable apps dir.
+ * If the apps dir is not writable, you can't download&install extra apps
+ * in the admin apps menu.
+ */
+"writable_appsdir" => true,
 // "datadirectory" => ""
 );
 ?>

lib/util.php

 		}
 		
 		// Check if apps folder is writable.
-		if(!is_writable(OC::$SERVERROOT."/apps/")) {
+		if(OC_Config::getValue('writable_appsdir', true) && !is_writable(OC::$SERVERROOT."/apps/")) {
 			$tmpl = new OC_Template( '', 'error', 'guest' );
 			$tmpl->assign('errors',array(1=>array('error'=>"Can't write into apps directory 'apps'",'hint'=>"You can usually fix this by giving the webserver user write access to the config directory in owncloud")));
 			$tmpl->printPage();