CMake: Fix absolute path detection on Windows

[squeek502]

Fixes build error on Windows introduced in #291

See https://ci.appveyor.com/project/racker-buildbot/luvi/branch/master/job/lbnl00gj7bvjexqy

if(NOT file MATCHES "^/") will fail to detect absolute paths on Windows (which typically start with a drive letter like C:/, but can also have a network path like \\server\share).

Instead, get_filename_component with REALPATH or IS_ABSOLUTE should be used.

[zhaozg]

+1