It would be useful if the fastfloat library had macros with the library version number.
Almost all other libraries provide that. For instance:

The Boost library collection ver. 1.85.0 has a header Version.hpp that contains
#define BOOST_VERSION 108500

The Eigen library ver. 3.4.0 has a header macros.hpp that contains
#define EIGEN_WORLD_VERSION 3
#define EIGEN_MAJOR_VERSION 4
#define EIGEN_MINOR_VERSION 0

In the FreeType library ver. 2.13.3 the main header freetype.h contains
#define FREETYPE_MAJOR 2
#define FREETYPE_MINOR 13
#define FREETYPE_PATCH 3

etc.