Add zlib warning to diagnose command

src/Composer/Command/DiagnoseCommand.php

@@ -494,6 +494,10 @@ EOT
             $warnings['apc_cli'] = true;
         }
 
+        if (!extension_loaded('zlib')) {
+            $warnings['zlib'] = true;
+        }
+
         ob_start();
         phpinfo(INFO_GENERAL);
         $phpinfo = ob_get_clean();
@@ -596,6 +600,12 @@ EOT
                         $displayIniMessage = true;
                         break;
 
+                    case 'zlib':
+                        $text  = 'The zlib extension is not loaded, this can slow down Composer a lot.'.PHP_EOL;
+                        $text .= 'If possible, enable it or recompile php with --with-zlib'.PHP_EOL;
+                        $displayIniMessage = true;
+                        break;
+
                     case 'sigchild':
                         $text  = "PHP was compiled with --enable-sigchild which can cause issues on some platforms.".PHP_EOL;
                         $text .= "Recompile it without this flag if possible, see also:".PHP_EOL;