iconv.h is needed in order to build zabbix_agentd. Currently configure will fail if it's not available in /usr/include or /usr/local/include. There is no way to specify path to it. Even if you specify "-I/custom/path/to/iconv" with CFLAGS like this:
env CFLAGS="-I/custom/path/to/iconv" ./configure --enable-agent
it will fail.