Cross compilation for OpenWrt is done with the OpenWrt build system on a Linux host machine:
https://openwrt.org/docs/guide-developer/build-system/install-buildsystem

https://openwrt.org/docs/guide-developer/build-system/install-buildsystem

Clone OpenWrt to some place in your home directory (<wrt dir>)

$ git clone https://git.openwrt.org/openwrt/openwrt.git
$ git clone https://github.com/badaix/snapos.git

cd openwrt
git pull
git checkout v21.02.2

Within the <wrt dir>/package directory create a symbolic link to <snapos dir>/openwrt:

$ cd packages
$ ln -s ../../snapos/openwrt ./snapcast

$ cd <wrt dir>
$ ./scripts/feeds update -a
$ ./scripts/feeds install -a

In make menuconfig select your Target System, Subtarget, Target Profile and navigate to Sound/snapcast and select Snapserver and/or Snapclient

$ cd <wrt dir>
$ make defconfig
$ make menuconfig
$ make -j $(nproc)

If there is an update for snapcast available, it can be rebuilt like this:

$ cd <wrt dir>
$ make package/snapcast/snapcast/clean
$ make package/snapcast/snapcast/compile

The packaged ipk files are in

<wrt dir>/bin/packages/<pkg_arch>/base/snapclient_x.x.x_<pkg_arch>.ipk
<wrt dir>/bin/packages/<pkg_arch>/base/snapserver_x.x.x_<pkg_arch>.ipk

https://openwrt.org/docs/guide-developer/using_the_sdk

Instead of building the entire image including snapcast and all dependencies, this methods uses the so called OpenWRT-SDK to only build the package and install it to an official vanilla OpenWRT build for your device.

• faster
• uses less disk space while building
• less prone to dependency related errors

• only works for systems with available official builds
• if SnapOS would optimize the base OpenWRT-System (currently it doesn't), these changes would not be reflected by this method

General remarks:

• At time of writing we use OpenWRT version 19.07.2.
• In this example snapcast is built for a TP-Link TL-WR710 v2.1.

Find your device's page, e.g. https://openwrt.org/toh/tp-link/tl-wr710n

download the latest image from there: http://downloads.openwrt.org/releases/19.07.2/targets/ar71xx/generic/openwrt-19.07.2-ar71xx-generic-tl-wr710n-v2.1-squashfs-factory.bin

note down your device's architecture:

• ar71xx/generic

Download the latest release of the SDK for your architecture and unpack:

$ wget https://downloads.openwrt.org/releases/19.07.2/targets/ar71xx/generic/openwrt-sdk-19.07.2-ar71xx-generic_gcc-7.5.0_musl.Linux-x86_64.tar.xz
$ tar -xf openwrt-sdk-19.07.2-ar71xx-generic_gcc-7.5.0_musl.Linux-x86_64.tar.xz

Within the <wrt-sdk dir>/package directory create a symbolic link to <snapos dir>/openwrt:

$ cd <wrt-sdk dir>
$ ln -s <snapos dir>/openwrt package/snapcast

$ ./scripts/feeds update -a
$ ./scripts/feeds install -a

In make menuconfig navigate to Sound/snapcast and select Snapclient

Also make sure to select your device in the target settings.

$ cd <wrt dir>
$ make defconfig
$ make menuconfig
$ make package/snapcast/compile

The packaged ipk file is in

<wrt dir>/bin/packages/<pkg_arch>/base/snapclient_x.x.x_<pkg_arch>.ipk

Assuming you already have installed OpenWRT on your device, you now have to copy the ipk to the device, e.g. with scp:

$ scp <wrt-sdk dir>/bin/packages/<pkg_arch>/base/snapclient_x.x.x_<pkg_arch>.ipk <openWRT-IP>:/tmp/

SSH to your device to install:

# update package manager to be able to install dependencies:
$ opkg update
# if you are using a usb soundcard you will need:
$ opkg install kmod-usb-audio
# finally install the copied package:
$ opkg install /tmp/snapclient_0.19.0_mips_24kc.ipk

set your default options, e.g.:

$ vi /etc/default/snapclient
  SNAPCLIENT_OPTS="-d -h <snapserver-ip> -s 3"

after a reboot you should be all set:

$ reboot