synergy: Update from version 1.6.3 to 1.7.4.

Quite a lot of upstream changes happened in between, so here are the
upstream changes:

https://github.com/synergy/synergy/releases/tag/1.7.0
https://github.com/synergy/synergy/releases/tag/v1.7.1-stable
https://github.com/synergy/synergy/releases/tag/v1.7.2-stable
https://github.com/synergy/synergy/releases/tag/v1.7.3-stable
https://github.com/synergy/synergy/releases/tag/v1.7.4-stable

The upstream project has stopped using cryptopp
(synergy/synergy@80343c4) and now uses OpenSSL instead (see issue
synergy/synergy#4313) so we no longer need our patch.

Also, the issue for the 'install' target has been closed as obsolete and
although they had such a target in the meantime, it has been removed
again in synergy/synergy@d1eb7a6, so let's not rely on it anymore and
remove the comment on the closed/obsolete issue.

Forcing support for XRandR in the CMakeLists.txt file isn't needed
anymore, because the issues mentioned in ea4afb73

 have been fixed
upstream.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Reported-by: devhell <"^"@regexmail.net>