gtk/3.x.nix: fix cross-compilation

[ghost]

Currently pkgsCross.aarch64-multiplatform.gtk3 fails for a few
reasons:

• gtk-doc does not deal well with cross compilation, so we disable
  the docs when cross-compiling.

• a few of the buildInputs are executed at runtime: gdk-pixbuf,
  glib, and (if waylandSupport==true) wayland-scanner, so we move
  those to nativeBuildInputs

• introspection data is broken for cross-compilation (see gobject-introspection: fix cross-compilation #166199)
  so we disable it with -Dintrospection=false.

• gtk3's post-install script, build-aux/meson/post-install.py, was
  written without any consideration at all for cross-compilation.
  So we need to substituteInPlace it in three places and also pass
  it a different PKG_CONFIG_PATH.

With this commit, and #166199 I am able to successfully build
pkgsCross.aarch64-multiplatform.gtk3 on an x86_64 host.

Many of the conditionals are written in slightly-nonstandard form (if
.. then null else ...) to ensure that this commit will not cause a
mass-rebuild of all the non-cross-compiled packages that depend on
gtk3.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux cross from x86_64-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/)
• 22.05 Release Notes (or backporting 21.11 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.

Currently pkgsCross.aarch64-multiplatform.gtk3 fails for a few
reasons:

• gtk-doc does not deal well with cross compilation, so we disable
  the docs when cross-compiling.

• a few of the buildInputs are executed at runtime: gdk-pixbuf,
  glib, and (if waylandSupport==true) wayland-scanner, so we move
  those to nativeBuildInputs

• introspection data is broken for cross-compilation (see gobject-introspection: fix cross-compilation #166199)
  so we disable it with -Dintrospection=false.

• gtk3's post-install script, build-aux/meson/post-install.py, was
  written without any consideration at all for cross-compilation.
  So we need to substituteInPlace it in three places and also pass
  it a different PKG_CONFIG_PATH.

With this commit, and #166199 I am able to successfully build
pkgsCross.aarch64-multiplatform.gtk3 on an x86_64 host.

Many of the conditionals are written in slightly-nonstandard form (if
.. then null else ...) to ensure that this commit will not cause a
mass-rebuild of all the non-cross-compiled packages that depend on
gtk3.

[ghost]

Closed in favor of #148618, which does a much better job.