libnet: fix build issue with package upgrade

The main issue was the endianess detection. The ac override did not work.
The current version has updated automake and autoconf files that address better endianess detection and it plays well with YP autotools.

Build tested on all qemus arches.

Signed-off-by: Armin Kuster <akuster@mvista.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>