add startup check for iconv (refs #571)

include/sanity_check.php

@@ -117,6 +117,10 @@
 				array_push($errors, "PHP support for ctype functions are required by HTMLPurifier.");
 			}
 
+			if (!function_exists("iconv")) {
+				array_push($errors, "PHP support for iconv is required to handle multiple charsets.");
+			}
+
 			if (ini_get("safe_mode")) {
 				array_push($errors, "PHP safe mode setting is not supported.");
 			}