Fix r561520

/!\ scilab-6.1.0_4: Makefile errors /!\

you cannot include bsd.port[.pre].mk twice

Defining both PORTVERSION and DISTVERSION is wrong, only set one, if necessary,
set DISTNAME

• Error code 1

Stop.
make: stopped in /usr/ports/math/scilab

Approved by: portmgr (blanket)