Does not really matter in this case, but good that you checked!
This time it’s #include <cstdint> in src/gromacs/utility/flags.h, between #define GMX_UTILITY_FLAGS_H and namespace gmx.
If you get any more issues, you can search the unknown symbol at C++ reference - cppreference.com, and then look for the phrase Defined in header at the top of the page. E.g., std::getenv - cppreference.com.
However, newer GROMACS version have it all fixed (at least the two issues you listed so far) and in general has many other improvements. Any reasons you prefer 2021?