gcc: always enable inhibit_libc=true for --without-headers builds

[trofi]

It's a follow-up to the breakage caused by 21966e1
("gcc: pass --with-build-sysroot=/"). It caused pkgsLLVM
cross-toolchain bootstrap breakage:

$ nix build --no-link -f. pkgsLLVM.hello
...
failed: /nix/store/...-x86_64-unknown-linux-gnu-stage-static-gcc-13.0.0.drv:
...
configure flags: ... --enable-languages=c --disable-multilib \
  --disable-shared --enable-plugin ... --disable-libssp --disable-nls \
  --without-headers --disable-threads --disable-libgomp --disable-libquadmath \
  --disable-shared --disable-libatomic --disable-decimal-float --disable-libmpx \
  --disable-bootstrap \
  \
  --build=x86_64-unknown-linux-gnu \
  --host=x86_64-unknown-linux-gnu \
  --target=x86_64-unknown-linux-gnu
...
The directory that should contain system headers does not exist:
  /usr/include
make[2]: *** [Makefile:3279: stmp-fixinc] Error 1 shuffle=1658621302
rm gfdl.pod gcc.pod gcov-dump.pod gcov-tool.pod fsf-funding.pod gpl.pod cpp.pod gcov.pod lto-dump.pod
make[2]: Leaving directory '/build/build/gcc'

Note: it's a no-libc build. It's not expected to use any libc headers.
But in this case fixincludes tries to run and uses default /usr/include
location.

Fixinsludes is not normally expected to run during cross-compilation
on --without-headers. gcc/configure.ac:

: ${inhibit_libc=false}
if { { test x$host != x$target && test "x$with_sysroot" = x ; } ||
       test x$with_newlib = xyes ; } &&
     { test "x$with_headers" = xno || test ! -f "$target_header_dir/stdio.h"; } ; then
       inhibit_libc=true
fi

The change explicitly passes inhibit_libc=true to configure to disable
include fixing on such cases.

Fixed nix build --no-link -f. pkgsLLVM.hello toolchain bootstrap.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[trofi]

Will keep in draft until get a few more tests finished.

[trofi]

Ready.

Fixed: pkgsLLVM.hello on x86_64-linux.
Tested (still works): hello, pkgsCross.powernv.stdenv.cc, pkgsCross.powernv.gcc, pkgsCross.mingw32.re2c, pkgsCross.or1k.stdenv.cc on x86_64-linux.
Tested (still works): hello, pkgsCross.powernv.stdenv.cc, pkgsCross.powernv.gcc

[trofi]

I'll merge it a bit quicker to get pkgsLLVM.* fixed on staging. This is a low risk change.