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
```