make: change to AC_CONFIG_SRCDIR([configure.ac]) since we moved config.h.in.
diff --git a/configure b/configure
index df775ad..c7d181b 100755
--- a/configure
+++ b/configure
@@ -595,7 +595,7 @@
 PACKAGE_BUGREPORT='ndn-lib@lists.cs.ucla.edu'
 PACKAGE_URL='https://github.com/named-data/ndn-cpp'
 
-ac_unique_file="config.h.in"
+ac_unique_file="configure.ac"
 # Factoring default headers for most tests.
 ac_includes_default="\
 #include <stdio.h>