logger_file_path, compile_dir, and cache_dir are assumed to be paths relative to the Piwik document root (ie PIWIK_INCLUDE_PATH). In 0.4, the relative path requirement was made explicit since earlier versions of Piwik allowed users to use an absolute path for compile_dir and cache_dir, but not logger_file_path. (inconsistency)
This change will permit the user to specify an absolute path (overriding the settings in global.ini.php); basically, we won't prefix with PIWIK_INCLUDE_PATH when an absolute path is specified.
(In ) Fixes #833 - Allow absolute path when relocating tmp folder outside of Piwik