Added a additional check if the posix extension is loaded

Added a check to prevent a thrown exception when the posix is not loaded in php.ini

Author: jdecoster

src/Symfony/Installer/NewCommand.php

@@ -364,7 +364,7 @@ protected function generateComposerProjectName()
 
         if (!empty($_SERVER['USERNAME'])) {
             $name = $_SERVER['USERNAME'].'/'.$name;
-        } elseif ($user = posix_getpwuid(posix_getuid())) {
+        } elseif (true === extension_loaded('posix') && $user = posix_getpwuid(posix_getuid())) {
             $name = $user['name'].'/'.$name;
         } elseif (get_current_user()) {
             $name = get_current_user().'/'.$name;