configure: Improve --with-linux* verbosity
configure: Improve --with-linux* verbosity

When --with-linux/-build/-release is used, display a message.
And warn if /lib/modules/<release>/ does not exist.

git-svn-id: 29c1264a5cf5e3532df57b06678e7347571c1a3c@2702 f1ba3bf5-cb5c-402b-92a9-7c6bdc83a356

ananos/open-mx,ananos/open-mx,ananos/xen2mx,ananos/xen2mx,ananos/xen2mx,ananos/open-mx,ananos/xen2mx