Add an is_absolute_path function to abstract out platform differences
in checking for an absolute or relative path.
Specifically fixes t4150-am on Windows.

[PT: updated following suggestion from j6t to support \* and //*]
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>