configure: Add a sanity check for tarballs without submodules
configure: Add a sanity check for tarballs without submodules

Because GitHub publishes broken tarballs on our behalf that we can't
disable, this adds a check that src/liblibc exists, and then
complains if not.

aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust