Assist getting more ports working on AMD64 by obeying the

Ports Collection documentation and use 'ARCH' rather than 'MACHINE_ARCH'.

math/plplot/Makefile

@@ -20,7 +20,7 @@ LIB_DEPENDS=	gd.4:${PORTSDIR}/graphics/gd
 USE_LIBLTDL=	yes
 USE_REINPLACE=	yes
 GNU_CONFIGURE=	yes
-CONFIGURE_TARGET=	--build=${MACHINE_ARCH}-portbld-freebsd${OSREL}
+CONFIGURE_TARGET=	--build=${ARCH}-portbld-freebsd${OSREL}
 CONFIGURE_ENV=	CPPFLAGS="-I${LOCALBASE}/include" \
 		LDFLAGS="-L${LOCALBASE}/lib" \
 		GDINCDIR="${LOCALBASE}/include" \