distro-build: split server and client parts
configure: propagate CPP, LD, CPPFLAGS, LDFLAGS for --enable-sdk build
subprocess: allocate for environment variables before fork

I'm running into stuck processes via `subprocess`, where the stuck
process is in malloc() for environment variables. I'm not really sure
of the problem, but moving that work before the fork() could
avoid problems related to pages or signal handlers.