libmariadb 10.2.x needs to be linked in together with iconv. On glibc
and musl iconv is part of libc. But on uclibc libiconv-full needs to be
used.

gnunet only has access to iconv on uclibc when BUILD_NLS is selected.
This commit adds hidden symbol GNUNET_HAS_ICONV_SUPPORT which sorts this
out.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>