opkg update
opkg install gcc -d ram

git clone https://git.openwrt.org/project/libubox.git

export PATH="/tmp/usr/bin:$PATH"
export LD_LIBRARY_PATH=/tmp/usr/lib:/usr/lib:/lib:.
export CPATH=/tmp/usr/include:.
export LIBRARY_PATH="$LD_LIBRARY_PATH"

gcc -fpic -c $(cat CMakeLists.txt | grep -i 'set.*sources' | sed -e 's/^[^ ]*//' -e 's/.$//')
gcc -shared -o ../libfoo.so *.o
gcc -Wall -o relayd main.c dhcp.c route.c -lfoo