ENH: forward download location to external projects

SuperBuild/CMake/External_boost.cmake

SuperBuild/CMake/External_curl.cmake

SuperBuild/CMake/External_expat.cmake

SuperBuild/CMake/External_fftw.cmake

SuperBuild/CMake/External_fltk.cmake

SuperBuild/CMake/External_gdal.cmake

SuperBuild/CMake/External_geos.cmake

SuperBuild/CMake/External_geotiff.cmake

SuperBuild/CMake/External_glew.cmake

SuperBuild/CMake/External_glfw.cmake

SuperBuild/CMake/External_glut.cmake

SuperBuild/CMake/External_ice.cmake

SuperBuild/CMake/External_itk.cmake

SuperBuild/CMake/External_jpeg.cmake

SuperBuild/CMake/External_libkml.cmake

SuperBuild/CMake/External_monteverdi.cmake

SuperBuild/CMake/External_monteverdi2.cmake

SuperBuild/CMake/External_muparser.cmake

SuperBuild/CMake/External_muparserx.cmake

SuperBuild/CMake/External_opencv.cmake