Handle relative paths in symlinks

Fixes #639

docs/about/release-notes.md

@@ -17,6 +17,7 @@ You can determine your currently installed version using `mkdocs --version`:
 ## Version 0.15.3 (2016-02-??)
 
 * Improve the error message the given theme can't be found.
+* Fix an issue with relative symlinks (#639)
 
 ## Version 0.15.2 (2016-02-08)
 

mkdocs/config/config_options.py

@@ -352,7 +352,8 @@ class Extras(OptionallyRequired):
                 # Some editors (namely Emacs) will create temporary symlinks
                 # for internal magic. We can just ignore these files.
                 if os.path.islink(fullpath):
-                    if not os.path.exists(os.readlink(fullpath)):
+                    fp = os.path.join(dirpath, os.readlink(fullpath))
+                    if not os.path.exists(fp):
                         continue
 
                 relpath = os.path.normpath(os.path.relpath(fullpath, docs_dir))