__TIME__ replacement might be empty depending on compiler

[firewave]

The %T format specifier we are using to generate the timestamp is not supported by all compilers:

/home/runner/work/simplecpp/simplecpp/simplecpp.cpp: In function ‘std::string getTimeDefine(const tm*)’:
/home/runner/work/simplecpp/simplecpp/simplecpp.cpp:3291:34: error: ISO C++98 does not support the ‘%T’ gnu_strftime format [-Werror=format=]
 3291 |     strftime(buf, sizeof(buf), "%T", timep);
      |                                  ^

[firewave]

It would be sufficient to have the placeholder (already used in code) in case it is not supported.