Use system Curl

This pull request will fix the issue, where Curl without c-ares will make Desura crash.

As the result there is no need to search for c-ares. We should use Curl whenever found in the system. This pull request is tested on Ubuntu. It is present in ppa:
https://launchpad.net/~makson96/+archive/desurium
for 2 months without any complains.

Together with this pull request please close: #617 #618 #711