This fixes spurious build-errors on OpenWrt, where the AM_ICONV macro
is undefined while invoking autoconfig. Later in the build, the ICONV
LDOPTIONS are set to @LIBICONV@, failing the build.
Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 9300a20dcce2217b121bd2020cc1a4ea41fa4475)