added search for zlib, because libpng 1.2 depends on it
added search for zlib, because libpng 1.2 depends on it

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