no need of add_custom_target for system libs

SuperBuild/CMake/External_boost.cmake

SuperBuild/CMake/External_curl.cmake

SuperBuild/CMake/External_expat.cmake

SuperBuild/CMake/External_fftw.cmake

SuperBuild/CMake/External_gdal.cmake

SuperBuild/CMake/External_geos.cmake

SuperBuild/CMake/External_geotiff.cmake

SuperBuild/CMake/External_itk.cmake

SuperBuild/CMake/External_libkml.cmake

SuperBuild/CMake/External_libsvm.cmake

SuperBuild/CMake/External_muparser.cmake

SuperBuild/CMake/External_muparserx.cmake

SuperBuild/CMake/External_opencv.cmake

SuperBuild/CMake/External_openjpeg.cmake

SuperBuild/CMake/External_openssl.cmake

SuperBuild/CMake/External_openthreads.cmake

SuperBuild/CMake/External_ossim.cmake

SuperBuild/CMake/External_otb.cmake

SuperBuild/CMake/External_png.cmake

SuperBuild/CMake/External_qt4.cmake