Fix mobius build with optional xvfb-run.

git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1609 b0c55286-4f34-0410-a049-a1e7e93b0762
fix typo

git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1610 b0c55286-4f34-0410-a049-a1e7e93b0762