Use AC_CONFIG_AUX_DIR([build-aux]).

This resolves #293.
diff --git a/configure.ac b/configure.ac
index 7a1290e..5a3eba2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,6 +1,8 @@
 dnl Process this file with autoconf to produce a configure script.
 AC_INIT([Makefile.in])
 
+AC_CONFIG_AUX_DIR([build-aux])
+
 dnl ============================================================================
 dnl Custom macro definitions.