build: just use autoreconf, avoid calling tools manually

patch by Jan Engelhardt (jengelh at medozas dot de)

fixes http://code.google.com/p/webp/issues/detail?id=59

Change-Id: I3fbb9124ce0b430b6340b3df5496e75cc5a9fedc

autogen.sh

@@ -1,5 +1,2 @@
 #! /bin/sh -e
-aclocal
-automake
-autoconf
-./configure "$@"
+exec autoreconf -fi