Do not require cairo to have PDF, EPS, SVG support

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Add configure option to disable rrd_restore support

For systems that do not have libxml2 installed

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Add configure option to disable rrdcached

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Add configure option to disable examples

Having examples installed is not always desired
(e.g. no use of them on embedded systems)

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Merge pull request #540 from maxnet/master

Loosen PDF, EPS, SVG, libxml2 dependency, and add --disable-feature options