build path entry using a UNC path

Don't mark absolute paths without device segment on Windows as relative.

Absolute Windows paths without device are possible if they are UNC -
based (starts with two leading back slashes like \\server\folder).

Change-Id: I48d8b22576b08cd66b1e2b2c18f74fb24de87fb8
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>