Log Message:
Greatly simplify the logic that tries to determine whether -lintl
needs -liconv in order to satisfy linkage requirements.  This is now
patterned after the approach taken with readline and termlib.

Examples on NetBSD for a package that includes only
gettext-lib/buildlink3.mk:

PREFER_NATIVE=	yes
PREFER_PKGSRC=	# empty
# This uses the native gettext and native iconv, with:
#	BUILDLINK_LDADD.gettext == "-lintl"

PREFER_NATIVE=	yes
PREFER_PKGSRC=	iconv
# This uses the native gettext and native iconv, with:
#	BUILDLINK_LDADD.gettext == "-lintl"

PREFER_NATIVE=	yes
PREFER_PKGSRC=	gettext
# This uses the pkgsrc gettext and native iconv, with:
#	BUILDLINK_LDADD.gettext == "-lintl"

PREFER_NATIVE=	yes
PREFER_PKGSRC=	gettext iconv
# This uses the pkgsrc gettext and pkgsrc iconv, with:
#	BUILDLINK_LDADD.gettext == "-lintl -liconv"

PREFER_NATIVE=	# empty
PREFER_PKGSRC=	yes
# This uses the pkgsrc gettext and pkgsrc iconv, with:
#	BUILDLINK_LDADD.gettext == "-lintl -liconv"

PREFER_NATIVE=	iconv
PREFER_PKGSRC=	yes
# This uses the pkgsrc gettext and native iconv, with:
#	BUILDLINK_LDADD.gettext == "-lintl"

PREFER_NATIVE=	gettext
PREFER_PKGSRC=	yes
# This uses the native gettext and native iconv, with:
#	BUILDLINK_LDADD.gettext == "-lintl"

PREFER_NATIVE=	gettext iconv
PREFER_PKGSRC=	yes
# This uses the native gettext and native iconv, with:
#	BUILDLINK_LDADD.gettext == "-lintl"