Added configure check for readlink.

diff --git a/configure.ac b/configure.ac
index 5ea663f..b641220 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1040,6 +1040,7 @@
 	popen \
 	pread \
 	pwrite \
+	readlink \
 	rfork \
 	select \
 	sendmsg \