Hack to select iconv implementation - libiconv vs. libc iconv (#15437)

etc/spack/defaults/packages.yaml

var/spack/repos/builtin/packages/apr-util/package.py

var/spack/repos/builtin/packages/bash/package.py

var/spack/repos/builtin/packages/dcmtk/package.py

var/spack/repos/builtin/packages/diffutils/package.py

var/spack/repos/builtin/packages/doxygen/package.py

var/spack/repos/builtin/packages/gcc/package.py

var/spack/repos/builtin/packages/gdal/package.py

var/spack/repos/builtin/packages/git/package.py

var/spack/repos/builtin/packages/glib/package.py

var/spack/repos/builtin/packages/gnupg/package.py

var/spack/repos/builtin/packages/gnuplot/package.py

var/spack/repos/builtin/packages/grass/package.py

var/spack/repos/builtin/packages/gtksourceview/package.py

var/spack/repos/builtin/packages/lftp/package.py

var/spack/repos/builtin/packages/libc/package.py

var/spack/repos/builtin/packages/libgd/package.py

var/spack/repos/builtin/packages/libiconv/package.py

var/spack/repos/builtin/packages/libspatialite/package.py

var/spack/repos/builtin/packages/libunistring/package.py