I think that they don't do too drastic changes. There are certainly checks in the compilation process to not accidentally link to /usr/lib/ libraries, but I thi

I think that they don't do too drastic changes. There are certainly checks in the compilation process to not accidentally link to /usr/lib/ libraries, but I think the main problem of finding libraries works with a simple compiler wrapper adding needed -Lflags (for finding libs at compile time) and RPATH entries to binaries (for finding libs at run time). There of course may be "hacks" made to gcc/libc but I don't see a reason they would be essential for functioning. One notable thing Nix does is hardcode a path to a specific dynamic linker (e.g. /nix/store/HASH-glibc-2.23/lib/ld-linux-x86-64.so.2). All other dynamic libraries should be referenced through soname+RPATH.