Switch to USES=iconv:build.

The port does not use iconv on its own, the only dependency comes via
libxml2, which has one header that includes iconv.h.

biology/libsbml/Makefile

@@ -3,6 +3,7 @@
 
 PORTNAME=	libsbml
 PORTVERSION=	5.11.4
+PORTREVISION=	1
 CATEGORIES=	biology devel
 MASTER_SITES=	SF/sbml/${PORTNAME}/${PORTVERSION}/stable
 DISTNAME=	libSBML-${PORTVERSION}-core-src
@@ -14,7 +15,7 @@ LICENSE=	LGPL21
 
 WRKSRC=		${WRKDIR}/${PORTNAME}-${PORTVERSION}
 
-USES=		cmake:outsource iconv
+USES=		cmake:outsource iconv:build
 USE_GNOME=	libxml2
 USE_LDCONFIG=	yes