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

- Do not require PDF, EPS and SVG support in Cairo

Major Linux distributions do have them compiled into Cairo by default, so not much difference for most users.
But users that choose to compile Cairo themselves without those options should be allowed to do so.

- Adds --disable-rrd-restore option

To build without XML import functionality on systems that do not have libxml2 installed

closes #539

- Adds --disable-rrdcached --disable-examples options

To not build and install those.