Add Icon to Executable and Restrict GNUCXX compiler options

Summary:
Icons missing from executable on windows.

Fix compilation on MSVC by restricting -W like compiler flags to GNUCXX

Reviewers: #kde_games, aacid

Reviewed By: aacid

Subscribers: aacid

Tags: #kde_games, #craft

Differential Revision: https://phabricator.kde.org/D24745