Prevent APC causing random fatal errors [fixes composer#264]

Update the phar stub to prevent APC attempting to run the composer classes through
the opcode cache on supported APC versions to avoid the random fatal "duplicate
class" errors at runtime as reported in composer#264.

Prior to APC version 3.0.12 this was a PHP_INI_SYSTEM directive, so output a
warning instead.

src/Composer/Compiler.php

@@ -212,6 +212,16 @@ private function getStub()
  * the license that is located at the bottom of this file.
  */
 
+// Avoid APC causing random fatal errors per https://github.com/composer/composer/issues/264
+if (extension_loaded('apc') && ini_get('apc.enable_cli') && ini_get('apc.cache_by_default') {
+  if (version_compare(phpversion('apc'), '3.0.12', '>=')) {
+    ini_set('apc.cache_by_default', 0);
+  } else {
+    print "Warning: APC <= 3.0.12 may cause fatal errors when running composer commands.".PHP_EOL;
+    print "Update APC, or set apc.enable_cli or apc.cache_by_default to 0 in your php.ini.".PHP_EOL;
+  }
+}
+
 Phar::mapPhar('composer.phar');
 
 EOF;