cmake: When there is no stdenv.glibc, still fix search paths.

Apply all of the fixups from preConfigure as well!

For testing purposes I added the following assert:

assert (stdenv ? glibc) -> (stdenv.glibc == stdenv.cc.libc);

To ensure behavior in the case there is a 'stdenv.glibc' (linux-only?)
wasn't changed, which passes in the configurations I could think of.

Not including that since it seems like a bad requirement moving forward.