glibc: fix cross builds on Darwin

[mroi]

When cross-compiling glibc, the postInstall attribute will use the build-platform version of localedef to generate locale information. This does not work when cross-compiling glibc on Darwin machines, because evaluating buildPackages.glibc will correctly lead to an unsupported platform error. Darwin machines cannot host glibc and thus cannot run the binaries.

My suggested fix ensures that the postInstall snippet which uses buildPackages.glibc is only included, when the build platform is Linux. On other platforms, the locale information will not be generated. This appears to be no problem in my cross compilation tool chain.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

[Mic92]

Should be ok to do so that for now. Eventually however we want to depend on LC_LANG=C.UTF-8 in more and more places, also we probably will start at the build time in which case the macOS stdenv would provide the locale. Long-term it might be possible to compile a cross-platform localedef as a proper fix: http://cgit.openembedded.org/openembedded-core/tree/meta/recipes-core/glibc/cross-localedef-native_2.29.bb?h=master

[mroi]

Thanks for your thoughts on the future goals, I will keep this in mind for later cross-toolchain work.

Also, I’ll just leave this here as a reference for later: macOS natively ships with a /usr/bin/localedef tool. Its arguments are not fully compatible with glibc’s version, but from the manpage, it appears to fulfil a similar purpose. But it may be the better option to properly cross-compile glibc’s own localedef.

[angerman]

This should be merged in release-19.03 as well; see #67433

[matthewbauer]

Cheryy picked in 3924b34