Motivation for this change

icu-config --cppflags returns an incorrect include path pointing to the out output while the includes are in the dev output.

$ nix-shell -p icu.dev --run "icu-config --cppflags"
-I/nix/store/5bp445w0c6l8ng28yrb3k4cw58wq8kcx-icu4c-59.1/include
$ ls /nix/store/5bp445w0c6l8ng28yrb3k4cw58wq8kcx-icu4c-59.1/include
ls: cannot access '/nix/store/5bp445w0c6l8ng28yrb3k4cw58wq8kcx-icu4c-59.1/include': No such file or directory

Fortunately, most nix builds are not affected by this bug because they get the correct include path via the NIX_CFLAGS_COMPILE variable. But this is still broken and can potentially lead to further breakage and suffering, so let's fix it. :)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-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/)
• Fits CONTRIBUTING.md.