Fix build on linux.
Fix build on linux.

uus169/CuraEngine,be3d/CuraEngine,foosel/CuraEngine,patrick3coffee/CuraTinyG,alex1818/CuraEngine,alephobjects/CuraEngine,markwal/CuraEngine,totalretribution/CuraEngine,daid/CuraCutEngine,pratikshashroff/pcura,uus169/CuraEngine,electrocbd/CuraEngine,Ultimaker/CuraEngine,markwal/CuraEngine,mspark93/CuraEngine,patrick3coffee/CuraTinyG,phonyphonecall/CuraEngine,derekhe/CuraEngine,mspark93/CuraEngine,alex1818/CuraEngine,phonyphonecall/CuraEngine,fxtentacle/CuraEngine,daid/CuraCutEngine,Jwis921/PersonalCuraEngine,electrocbd/CuraEngine,be3d/CuraEngine,alex1818/CuraEngine,foosel/CuraEngine,derekhe/CuraEngine,jacobdai/CuraEngine-1,alephobjects/CuraEngine,mspark93/CuraEngine,uus169/CuraEngine,alephobjects/CuraEngine,ROBO3D/CuraEngine,pratikshashroff/pcura,be3d/CuraEngine,foosel/CuraEngine,markwal/CuraEngine,derekhe/CuraEngine,ROBO3D/CuraEngine,electrocbd/CuraEngine,phonyphonecall/CuraEngine,pratikshashroff/pcura,Jwis921/PersonalCuraEngine,patrick3coffee/CuraTinyG,ROBO3D/CuraEngine,Jwis921/PersonalCuraEngine,fxtentacle/CuraEngine,jacobdai/CuraEngine-1,totalretribution/CuraEngine,totalretribution/CuraEngine,jacobdai/CuraEngine-1,fxtentacle/CuraEngine,Ultimaker/CuraEngine