cmake: we need at least libcurl 7.19.1

fixes #661
Merge pull request #662 from karolherbst/curl_req

cmake: we need at least libcurl 7.19.1
Tinyxml2::LoadFile returns 0 for success instead of true thus updater is broken #665
Merge pull request #666 from lodle/bugfix665

Tinyxml2::LoadFile returns 0 for success instead of true thus updater is broken #665