GNAT requires the C library developer’s package to be installed. The name of of that package depends on your GNU/Linux distribution:
glibc-devel;
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 following packages:
glibc.i686, glibc-devel.i686, ncurses-libs.i686
libc6:i386, libc6-dev:i386, lib32ncursesw5
Other GNU/Linux distributions might be choosing a different name for those packages.