Syncplay is quite outdated at 1.3.4 with the current version being 1.5.1.

I already tested 1.5.1 and it works fine, but the build process has changed a bit. A simple %make_install is now enough to install icons, desktop files and syncplay itself, the other workaround with manual installation and sed and so forth are not necessary anymore.

Especially DESTDIR no works as intended. Therefor the %make_install.