Invocation of openscad with an absolute path failed:
  ./openscad /home/username/foo.scad
.. because the curent working directory was always prepended; so
internally, it attempted to open $PWD + /home/username/foo.scad

Use platform aware QDir functionality to only prepend the absolute
current working directory prefix, if the given path is relative.
Merge pull request #521 from hzeller/fix-broken-absolute-path

Fix invocation with absolute path.