When building Siemens Jailhouse on Ubuntu 20.04 got -Werror-implicit-function-declaration errors

When building Siemens Jailhouse on Ubuntu 20.04 (x86_64) with target ARCH=arm64 like this:

$ sudo make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-

I am getting a lot of errors of the type:

[-Werror-implicit-function-declaration]

I did

export CFLAGS="-Wno-error"

but it does not change anything. In the /jailhouse/Makefile I could not find where I can set CFLAGS. Even if I try to pass it to the make call, still getting errors. I tried $ make -n and I can see that '-Werror-implicit-function-declaration' is passed to CC but I do not know how to disable it.

Your help is appreciated!