Hi!
A related question was answered in another threads: New Intel icx and icpx compilers | use of undeclared identifier 'getenv' - #4 by al42and
You should modify src/gromacs/utility/flags.h by adding a new line containing #include <cstdint> at the top of the file, between #define GMX_UTILITY_FLAGS_H and namespace gmx.
Better yet, use newer GROMACS which has this issue fixed, like 2024.4. The 2021.x series is not supported, and newer versions have better performance and less bugs.