Partially revert PR #2190

The core shouldn't depend on constants that are not defined by itself
diff --git a/index.php b/index.php
index 6a93242..8008211 100755
--- a/index.php
+++ b/index.php
@@ -68,7 +68,7 @@
 	default:
 		header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
 		echo 'The application environment is not set correctly.';
-		exit(1); // EXIT_* constants not yet defined; 1 is EXIT_ERROR, a generic error.
+		exit(1); // EXIT_ERROR
 }
 
 /*
@@ -192,7 +192,7 @@
 	{
 		header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
 		echo 'Your system folder path does not appear to be set correctly. Please open the following file and correct this: '.pathinfo(__FILE__, PATHINFO_BASENAME);
-		exit(3); // EXIT_* constants not yet defined; 3 is EXIT_CONFIG.
+		exit(3); // EXIT_CONFIG
 	}
 
 /*
@@ -228,7 +228,7 @@
 		{
 			header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
 			echo 'Your application folder path does not appear to be set correctly. Please open the following file and correct this: '.SELF;
-			exit(3); // EXIT_* constants not yet defined; 3 is EXIT_CONFIG.
+			exit(3); // EXIT_CONFIG
 		}
 
 		define('APPPATH', BASEPATH.$application_folder.DIRECTORY_SEPARATOR);
@@ -245,7 +245,7 @@
 		{
 			header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
 			echo 'Your view folder path does not appear to be set correctly. Please open the following file and correct this: '.SELF;
-			exit(3); // EXIT_* constants not yet defined; 3 is EXIT_CONFIG.
+			exit(3); // EXIT_CONFIG
 		}
 		else
 		{