[gobject-introspection/paths.win] giscanner/girwriter.py: Fix running on Windows

[Chun-wei Fan <fanchunwei src gnome org>]

commit aaa7af50f98771efc8172c5dff7a898feda8c423
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Thu Dec 6 17:45:12 2018 +0800

    giscanner/girwriter.py: Fix running on Windows
    
    ... When we are building in a drive that is different from the drive
    that we are acquiring the depedencies from.  For example,
    os.path.relpath() does not like it when we we are building G-I on D:\foo
    while the GLib headers are found are C:\glib.

 giscanner/girwriter.py | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
---
diff --git a/giscanner/girwriter.py b/giscanner/girwriter.py
index 4f58229b..6bbb054d 100644
--- a/giscanner/girwriter.py
+++ b/giscanner/girwriter.py
@@ -127,7 +127,14 @@ class GIRWriter(XMLWriter):
     def _get_relative_path(self, filename):
         res = filename
         for root in self.sources_roots:
-            relpath = os.path.relpath(filename, root)
+            relpath = ''
+            try:
+                relpath = os.path.relpath(filename, root)
+
+            # We might be on different drives on Windows, so relpath() won't work
+            except ValueError:
+                relpath = filename
+
             if len(relpath) < len(res):
                 res = relpath