COMP: If SlicerExtension, Use BatchMake CURL_SPECIAL_LIBZ var to compile bmcurl with zlib from Slicer because tries to link with zlib from Slicer anyway (names mangled for Slicer: slicer_zlib_... see SlicerBuildTree/zlib-install/include/zlib_mangle.h). Otherwise compiles with zlib from ITK (mangled itk_zlib_... see BatchMake/Utilities/Zip/itk_zlib_mangle.h) and try to link with zlib from Slicer => FAIL