--- Configure	2018-06-04 16:58:27.000000000 +0200
+++ Configure.new	2018-06-04 17:09:17.744789676 +0200
@@ -1209,7 +1209,7 @@
     open(PIPE, "$cc -dM -E -x c /dev/null 2>&1 |");
     while (<PIPE>) {
 	m/^#define\s+(\w+(?:\(\w+\))?)(?:\s+(.+))?/ or last;
-	$predefined{$1} = $2 // "";
+	$predefined{$1} = defined($2) ? $2 : "";
     }
     close(PIPE);