Cross-build fails if host has no libpng

Project MESSAGE: Running: /tmp/e2/build/build/qtwebengine/src/3rdparty/chromium/tools/gn/out/Release/gn gen /tmp/e2/build/build/qtwebengine/src/core/release --script-executable=/usr/bin/python2 --args='use_qt=true is_component_build=false is_shared=true enable_nacl=false enable_remoting=false enable_reporting=false enable_web_speech=false use_allocator_shim=false use_allocator="none" v8_use_external_startup_data=false treat_warnings_as_errors=false enable_swiftshader=false use_custom_libcxx=false use_jumbo_build=true jumbo_file_merge_limit=50 enable_basic_printing=false enable_print_preview=false enable_pdf=false enable_plugins=false enable_widevine=false enable_spellcheck=true enable_webrtc=false enable_precompiled_headers=true symbol_level=0 remove_webcore_debug_symbols=true remove_v8base_debug_symbols=true v8_use_snapshot=true use_cups=false use_gconf=false use_gio=false use_gnome_keyring=false use_kerberos=false linux_use_bundled_binutils=false use_nss_certs=true use_openssl_certs=false is_clang=false target_sysroot="/opt/emlix/test/sysroot" arm_tune="cortex-a9" arm_float_abi="hard" arm_version=7 arm_use_neon=true custom_toolchain="/tmp/e2/build/build/qtwebengine/src/toolchain:target" host_toolchain="/tmp/e2/build/build/qtwebengine/src/toolchain:host" v8_snapshot_toolchain="/tmp/e2/build/build/qtwebengine/src/toolchain:v8_snapshot" target_cpu="arm" pkg_config="/tmp/e2/build/build/qtwebengine/src/core/pkg-config_wrapper.sh" host_pkg_config="/tmp/e2/build/build/qtwebengine/src/core/pkg-config-host_wrapper.sh" use_system_libpng=true use_system_libjpeg=true use_system_freetype=true use_glib=false use_pulseaudio=false use_alsa=true use_system_libffi=true use_libpci=false use_xscrnsaver=false is_desktop_linux=false use_gold=false use_ozone=true use_sysroot=false enable_session_service=false ozone_auto_platforms=false ozone_platform_headless=true ozone_platform_external=true ozone_platform="qt" toolkit_views=false is_debug=false qtwebengine_target="/tmp/e2/build/build/qtwebengine/src/core/release:QtWebEngineCore"' --root=/tmp/e2/build/qt-everywhere-src-5.10.1/qtwebengine/src/3rdparty/chromium 
[...]
ERROR at //build/config/linux/pkg_config.gni:103:17: Script returned non-zero exit code.
    pkgresult = exec_script(pkg_config_script, args, "value")
                ^----------
Current dir: /tmp/e2/build/build/qtwebengine/src/core/release/
Command: /usr/bin/python2 -- /tmp/e2/build/qt-everywhere-src-5.10.1/qtwebengine/src/3rdparty/chromium/build/config/linux/pkg-config.py -p /tmp/e2/build/build/qtwebengine/src/core/pkg-config-host_wrapper.sh libpng
Returned 1.
stderr:

Package libpng was not found in the pkg-config search path.
Perhaps you should add the directory containing `libpng.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libpng' found
Could not run pkg-config.

See //third_party/libpng/BUILD.gn:126:3: whence it was called.
  pkg_config("system_libpng") {
  ^----------------------------
See //tools/imagediff/BUILD.gn:22:7: which caused the file to be included.
      "//third_party/libpng",
      ^---------------------
Project ERROR: GN run error!