threading.hxx: Add preprocessor check for clang support of std threading.
threading.hxx: Add preprocessor check for clang support of std threading.

xiwei-zhang/vigra,timoMa/vigra,xiwei-zhang/vigra,dagophil/vigra,funkey/vigra,funkey/vigra,funkey/vigra,dstoe/vigra,cha007/vigra,ThomasWalter/vigra,ThomasWalter/vigra,hmeine/vigra,stuarteberg/vigra,capturePointer/vigra,stuarteberg/vigra,mmuellner/vigra,cha007/vigra,DaveRichmond-/vigra,cha007/vigra,psteinb/vigra,timoMa/vigra,mmuellner/vigra,mmuellner/vigra,stuarteberg/vigra,hmeine/vigra,dagophil/vigra,dstoe/vigra,DaveRichmond-/vigra,stuarteberg/vigra,mmuellner/vigra,cha007/vigra,mmuellner/vigra,stuarteberg/vigra,ThomasWalter/vigra,dstoe/vigra,DaveRichmond-/vigra,psteinb/vigra,psteinb/vigra,hmeine/vigra,capturePointer/vigra,kod3r/vigra,kod3r/vigra,cha007/vigra,cha007/vigra,timoMa/vigra,dagophil/vigra,ThomasWalter/vigra,stuarteberg/vigra,capturePointer/vigra,funkey/vigra,DaveRichmond-/vigra,DaveRichmond-/vigra,capturePointer/vigra,ThomasWalter/vigra,kod3r/vigra,dstoe/vigra,hmeine/vigra,timoMa/vigra,mmuellner/vigra,mmuellner/vigra,xiwei-zhang/vigra,capturePointer/vigra,psteinb/vigra,psteinb/vigra,psteinb/vigra,dstoe/vigra,capturePointer/vigra,dagophil/vigra,ThomasWalter/vigra,psteinb/vigra,kod3r/vigra,dstoe/vigra,funkey/vigra,funkey/vigra,timoMa/vigra,DaveRichmond-/vigra,hmeine/vigra,kod3r/vigra,xiwei-zhang/vigra,DaveRichmond-/vigra,dagophil/vigra,funkey/vigra,timoMa/vigra,xiwei-zhang/vigra,ThomasWalter/vigra,timoMa/vigra,xiwei-zhang/vigra,capturePointer/vigra,dagophil/vigra,hmeine/vigra,kod3r/vigra,hmeine/vigra,cha007/vigra,dagophil/vigra,stuarteberg/vigra,xiwei-zhang/vigra,dstoe/vigra,kod3r/vigra