To allow flags set on the builder with Build::flag/Build::flag_if_supported to override other flags, and to allow CFLAGS to override all other flags.

Found while doing #1401.

Now the order of flags are as follows, with later flags overriding previous ones. I've also here shown who "controls" which flag, to make it clear why we want the ordering to be like this.

1. Default flags
   • Controlled by cc-rs.
2. rustc-inherited flags
   • Controlled by rustc.
3. Builder flags
   • Controlled by the developer using cc-rs in e.g. their build.rs.
4. Environment flags
   • Controlled by the end user.