nix-cc.sh hardcoded flags

[ailrst]

Its not clear how to set compiler flags, hardcoding -O3 is not ideal, the configurability of liftmake.sh would be nice.

[katrinafyi]

I think this is difficult to fix given the implementation of nix-cc.sh. If we build inside the sandbox, then we are subject to a lot of its defaults.

But there are ways to influence the flags. Maybe worth looking into.