taking away this flag, the build works in both versions
Created attachment 366930 [details] environment (2.3.7)
Created attachment 366932 [details] build.log (2.3.7)
Created attachment 366934 [details] emerge --info '=dev-games/clanlib-2.3.7::gentoo'
It was fixed now after 'emerge -e world' 2 weeks ago, because of changing my desktop profile. Don't know what was wrong, there was no specific output.