improve portability

[goatshriek]

Improve portability by:

• ensuring all variable declarations are at the beginning of the function
• ensure that stumpless.h is not included in any library sources
• support builds where gmtime is not available
• make the default log file name a configuration variable
• remove fuzz test use of C++ string class

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.54%. Comparing base (e262905) to head (c4921cd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## latest #410 +/- ## ======================================= Coverage 90.54% 90.54% ======================================= Files 46 46 Lines 4188 4188 Branches 556 556 ======================================= Hits 3792 3792 Misses 273 273 Partials 123 123 ```

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