autogen.sh: use #!/bin/sh instead of #!/bin/bash

We don't have any bashisms in this file, so we may as well use sh.

https://bugzilla.gnome.org/show_bug.cgi?id=722530

autogen.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 # Run this to generate all the initial makefiles, etc.
 
 srcdir=`dirname $0`