Should output "cargo:rerun-if-env-changed=..." for all variables that affect compilation

[glandium]

Currently, if you have a build script that uses cc, and alter, one of, say, CC, CXX, CFLAGS, etc. nothing happens. Worse yet, cargo happily outputs warnings from a previous build with the old environment variables, making you think something was rebuilt when that's not the case.

[alexcrichton]

I believe this is a duplicate of #230, so closing