Ticket 4399 full paths are not excluded

Hello all,

Here is a fix for the ticket 4399 (http://trac.cppcheck.net/ticket/4399), tested both on Linux and Windows. Detection of absolute paths on Windows is implemented as described in,

http://msdn.microsoft.com/en-us/library/aa365247.aspx#fully_qualified_vs._relative_paths

Please see the ticket for my reasons to avoid PathIsRelative().

Thanks for commenting and considering to merge.

Cheers,