You do this by invoking CC=gcc ./configure --host=i686-gnu $OTHER_FLAGS — i.e., we're building for the Hurd, but do your stupid checks using my host toolchain for now so that we can pretend that everything works and at least get something done today
• After this, you can actually build enough of glibc to install its headers. You'll also have to build csu/crt[1in].o & manually copy them over into the prefix, and also create a couple of empty files (include/gnu/stubs.h & the very lib/libc.so!)