gettext-full: avoid using iconv for host builds

gettext-full: avoid using iconv for host builds

The gettext-full host build might pick up iconv-stub host build  headers
during the build, leading to stray linker errors with unresolved references
to libiconv_open(), libiconv() and libiconv_close().

Since we're not needing iconv support on the host, pass the appropriate
cache variables to configure to prevent detection and linking of iconv.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>