fix compile

git-svn-id: https://geuz.org/svn/gmsh/trunk@17803 59c8cabf-10d6-4a28-b43f-3936b85956c4
occ requires -lrt on linux

git-svn-id: https://geuz.org/svn/gmsh/trunk@17804 59c8cabf-10d6-4a28-b43f-3936b85956c4