Fix for older versions of intel compiler
Fix for older versions of intel compiler

The intel-17 and intel-18 compilers seem to require that `u` be `const`:
```
/src/fmt/format.h(226): warning #437: reference to local variable of enclosing function is not allowed
        char data[sizeof(u)];
```
If `u` is declared as `const auto u =1u` instead of just `auto u=1u`, the file compiles with no warnings.
cppformat/cppformat,cppformat/cppformat,alabuzhev/fmt,alabuzhev/fmt,alabuzhev/fmt,cppformat/cppformat