The custom code returns the overlay instead of all packages but we
can sort-of fake it by filtering the resulting package set.

The custom code doesn't add the new packages to pkgsBuildBuild and it
doesn't create spliced packages for them so it won't work for cross
compiling.

It still doesn't work because the new packages are only added to pkgs
but not pkgs.pkgsBuildBuild etc so callPackage doesn't pass them to the
packages.

Parts of the old buildPhase actually belong in unpack or configure.

- GCC build was failing with warnings/errors for printf. NixOS disables
  hardening for format and pie for gcc so we do the same.
- gdb is built with Python support. The configure check for Python also
  uses -lncurses so we need that as well. The message is rather
  confusing because it claims not to find Python.