ENH: refactor: use macros for SB variables

CMake/External_GDAL.cmake

CMake/External_ITK.cmake

CMake/External_OpenCV.cmake

CMake/External_OpenJpeg.cmake

CMake/External_OpenThreads.cmake

CMake/External_SQLite.cmake

CMake/External_TinyXML.cmake

CMake/External_curl.cmake

CMake/External_expat.cmake

CMake/External_fftw.cmake

CMake/External_geos.cmake

CMake/External_geotiff.cmake

CMake/External_jpeg.cmake

CMake/External_libKML.cmake

CMake/External_libpng.cmake

CMake/External_muParser.cmake

CMake/External_muParserX.cmake

CMake/External_ossim.cmake

CMake/External_pcre.cmake

CMake/External_proj4.cmake