Autotools: Do not use source to call an another script

* Script is valid for bash but not any /bin/sh
* Thanks to Xiaofan Chen for the bug report
http://sourceforge.net/mailarchive/message.php?msg_id=29217871
diff --git a/autogen.sh b/autogen.sh
index 5872540..a4b87e4 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -1,3 +1,3 @@
 #!/bin/sh
-source ./bootstrap.sh
+./bootstrap.sh
 ./configure --enable-maintainer-mode --enable-examples-build $*