Remove hardcoded Guile executable path.

* scripts/haunt: Now a build artifact.
* scripts/haunt.in: New file.
* configure.ac: Pre-process haunt.in.

scripts/haunt.in

@@ -1,4 +1,4 @@
-#!/usr/bin/guile --no-auto-compile
+#!@GUILE@ --no-auto-compile
 -*- scheme -*-
 !#
 ;;; Haunt --- Static site generator for GNU Guile