Reconfiguring should preserve CFG_* settings

[erickt]

As I mentioned in graydon/rust#368, I need to specify a custom version of llvm and clang for compiling rust. Unfortunately, when the makefiles are changed, this triggers a reconfigure, which blows away my CFG_* settings. This results in the wrong version of clang being used, which causes my build to error out. Could the CFG_ be preserved across reconfigures to prevent this from happening?

[eholk]

The way I deal with needing a special version of LLVM and Clang is to create an env.sh that I source in my terminal before beginning rust work. This changes to PATH so that the Rust-compatible LLVM and Clang are preferred over the system default.

Of course, being able to specify a user config file that overrides the automatically determined values would be nice sometimes too.

[graydon]

overriding CFG settings manually is not supported; landed support for hitting this particular case in different way