I have attached the compile-log.
Created attachment 4047 [details] arts compile errors log
This looks at first sight like a make -j2 problem - see lines 1417, 1423 in the emerge log. To test this possibility try: MAKEOPTS="-j1" emerge arts
To danarmak: MAKEOPTS="-j1" helped. I could compile arts.
OK, then I'll add MAKEOPTS=-j1 to the arts-1.0.3 ebuild. Even though noone else reported this, better to compile a little slower (and it's a small package after all) than to risc compile problems. Please test the 3.1 tree (arts-1.1.0) for this problem. For now I'm adding MAKEOPTS=-j1 to arts-1.0.3 only.