GNAT requires the C library developer’s package to be installed. The name of of that package depends on your GNU/Linux distribution:
libc6-dev(normally installed by default).
If using the 32-bit version of GNAT on a 64-bit version of GNU/Linux, you’ll need the 32-bit version of the glibc and glibc-devel packages:
Other GNU/Linux distributions might be choosing a different name for that package.