How to compile from sources on openWRT?

I need CUPS on my router. But i have only sources.
Trying install gcc on Mikrotik RBM 33G.
Not enough free space...
i plug MicroSD card and mount it on /opt.
How to install gcc to /opt or how to move usr or root to sd?
Guess it wrong way.... Could i add cups to OpenWRT builder?
PLease give me advice.

You can select CUPS in menuconfig.

i dont have CUPS in menuconfig
./scripts/feeds update -a
./scripts/feeds install -a
OpenWRT 18.06.1