`ios::hexfloat` shouldn't be used

[frederick-vs-ja]

This line attempts to use the non-conforming extension std::ios::hexfloat.

https://github.com/panzi/formatstring/blob/8c7cb73cd69e29c3289924de7bb51e28cd032f79/src/formatvalue.cpp#L563

We should use the standard manner, i.e., replacing it with std::ios::fixed | std::ios::scientific. See also cppreference.

Also, conditional compilation shouldn't be used because such ability is added in C++11.