Being explicit about xrender support

Xrender support is needed to build the webkit module.

The default value for xrender is auto, so if there is no xrender it silently continues to build.
QT is build fine, but wkhtmltopdf fails because of no webkit support.

Be being explicit we make sure webkit will be installed, if xrender support can't be enabled, it will fail and save time.
Merge pull request #26 from renan/patch-2

Being explicit about xrender support