mkdirs() did always create a relative directory starting from the CWD.
When tmpdir is given an absolute path this resulted in a directory like
/home//path/to/tmpdir being created.
The actual cache directory is considered properly, though.
The fix detects whether the given tmpdir is an absolute path.
At least on Unix.
**NOTE:** I don't think this works correctly on Windows. Is there the need for an improvement?
mkdirs() did always create a relative directory starting from the CWD. When tmpdir is given an absolute path this resulted in a directory like