You could try passing -static to the linker (via LDFLAGS), but be aware that full static linking is not supported by glibc anymore (resp. it needs a glibc build which supports static linking).
This is due to the fact that nss libraries (name server switch) will be loaded dynamically (unless you compile your own glibc - but that defeats the purpose of nss). This might be enough for you however to reduce dependencies against system libraries.
But I could assume that a statically linked gcc is fairly huge - this might result in long startup times.
If your objective is only to make a relocatable toolchain, statically link with expat, gmp, mpfr and mpc should enough. You can simply apply https://patchwork.ozlabs.org/patch/359841/
