Pass --cfg=bootstrap for proc_macros or build scripts built by stage0
Cargo ignores RUSTFLAGS when building proc macro crates. However, sometimes rustc_macro needs to have conditional compilation when there are breaking changes to the `libproc_macro` API (see for example tell the difference between stage 0 and stage 1. Another alternative is to unconditionally build rustc_macros with the master libstd instead of the beta one (i.e. use `--sysroot stage0-sysroot`), but that led to strange and maddening errors: ``` error[E0460]: found possibly newer version of crate `std` which `proc_macro2` depends on --> /home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/tracing-attributes-0.1.13/src/lib.rs:90:5 | 90 | use proc_macro2::TokenStream; | ^^^^^^^^^^^ | = note: perhaps that crate needs to be recompiled? = note: the following crate versions were found: crate `std`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-b3602c301b71cc3d.rmeta crate `proc_macro2`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-rustc/release/deps/libproc_macro2-a83c1f01610c129e.rlib ```
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