this one has some special logic for meson cross-compile:

  strictDeps = true;

  depsBuildBuild = [
    pkg-config
  ];

  nativeBuildInputs = [
    meson
    ninja
    pkg-config
    gobject-introspection
    gtk-doc
    docbook-xsl-nons
    docbook_xml_dtd_43
  ]
  ++ lib.optionals (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) [
    mesonEmulatorHook
  ];

  buildInputs = [
    glib
  ];

it tried to use the following qemu wrapper:

#! /nix/store/mlbapfqmfrar7b87hmv2s9vb3g9dq3p7-bash-interactive-5.3p3/bin/bash
# shellcheck shell=bash

# fixes issues like
# libdbusmenu-gtk3-aarch64-unknown-linux-gnu> /build/libdbusmenu-16.04.0/libdbusmenu-gtk/tmp-introspect2jhtiwwn/.libs/DbusmenuGtk3-0.4:
# error while loading shared libraries: libdbusmenu-glib.so.4: cannot open shared object file: No such file or directory
# in non-meson builds

# see: https://github.com/void-linux/void-packages/blob/master/srcpkgs/gobject-introspection/files/g-ir-scanner-qemuwrapper
# https://github.com/openembedded/openembedded-core/blob/c5a14f39a6717a99b510cb97aa2fb403d4b98d99/meta/recipes-gnome/gobject-introspection/gobject-introspection_1.72.0.bb#L74
while read -r d; do \
    # some meson projects may have subprojects which use makefiles for docs(e.g. gi-docgen), ignore those as they will never be needed
    if [[ -f "$d/Makefile" && "$d" != *"subproject"* ]]; then
        GIR_EXTRA_LIBS_PATH="$(readlink -f "$d/.libs"):$GIR_EXTRA_LIBS_PATH"
        export GIR_EXTRA_LIBS_PATH
    fi
done < <(find "$NIX_BUILD_TOP" -type d)

# quoting broke the build of atk
# shellcheck disable=2086
exec /nix/store/w74a7hgymrh13gknfm07nmbs8nnzx625-qemu-user-10.1.2/bin/qemu-ppc64 ${GIR_EXTRA_OPTIONS:-} \
    ${GIR_EXTRA_LIBS_PATH:+-E LD_LIBRARY_PATH="${GIR_EXTRA_LIBS_PATH}"} \
    "$@"

if I disable modemmanager, it fails on gdk-pixbuf:

gdk-pixbuf> /nix/store/w74a7hgymrh13gknfm07nmbs8nnzx625-qemu-user-10.1.2/bin/qemu-ppc64: error while loading shared libraries: /nix/store/pyzyni2iaxjq6cw58ffyr70dcf1ydav7-glib-powerpc64-unknown-linux-gnuabielfv1-2.86.3/lib/libglib-2.0.so.0: ELF file data encoding not little-endian

i'm failing in pkgsCross.ppc64-elfv1.capnproto with pkgsCross.ppc64-elfv1.clangStdenv.cc being non-functional.

capnproto> -- Check for working CXX compiler: /nix/store/dw2m30ah26gl0rkxk585v85j5lh6sw8s-powerpc64-unknown-linux-gnuabielfv1-clang-wrapper-21.1.7/bin/powerpc64-unknown-linux-gnuabielfv1-clang++ - broken
[...]
capnproto>     Building CXX object CMakeFiles/cmTC_abe92.dir/testCXXCompiler.cxx.o
capnproto>     /nix/store/dw2m30ah26gl0rkxk585v85j5lh6sw8s-powerpc64-unknown-linux-gnuabielfv1-clang-wrapper-21.1.7/bin/powerpc64-unknown-linux-gnuabielfv1-clang++   -std=c++20  -MD -MT CMakeFiles/cmTC_abe92.dir/testCXXCompiler.cxx.o -MF CMakeFiles/cmTC_abe92.dir/testCXXCompiler.cxx.o.d -o CMakeFiles/cmTC_abe92.dir/testCXXCompiler.cxx.o -c /build/source/build/CMakeFiles/CMakeScratch/TryCompile-idO2pp/testCXXCompiler.cxx
[with -v added to clang shows its internal invocations]
capnproto>      "ld" --hash-style=gnu --eh-frame-hdr -m elf64ppc 
capnproto>     clang++: error: unable to execute command: posix_spawn failed: No such file or directory
capnproto>     clang++: error: linker command failed with exit code 1 (use -v to see invocation)