Use TMPDIR instead of cwd for temporary files during construction

[cartoonist]

In cache_config constructor, the default directory for temporary files is initialised by current working directory (./). It would be more convenient if it uses the value of TMPDIR environment variable to set the default directory.

[cartoonist]

I would like to add a minor comment; isn't this line redundant:

    tMSS file_map;

Since it is not used later in the function.