ENH: superbuild: use a specific variable for install prefix

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_jpeg.cmake

Superbuild/CMake/External_libkml.cmake

Superbuild/CMake/External_muparser.cmake

Superbuild/CMake/External_muparserx.cmake

Superbuild/CMake/External_opencv.cmake

Superbuild/CMake/External_openjpeg.cmake

Superbuild/CMake/External_openthreads.cmake

Superbuild/CMake/External_ossim.cmake

Superbuild/CMake/External_pcre.cmake

Superbuild/CMake/External_png.cmake

Superbuild/CMake/External_proj.cmake

Superbuild/CMake/External_qt4.cmake