Restore compatibility with rustc 1.15-1.16

Author: dtolnay

Cargo.toml

@@ -8,6 +8,7 @@ keywords = ["macros"]
 repository = "https://github.com/alexcrichton/proc-macro2"
 homepage = "https://github.com/alexcrichton/proc-macro2"
 documentation = "https://docs.rs/proc-macro2"
+build = "build.rs"
 description = """
 A stable implementation of the upcoming new `proc_macro` API. Comes with an
 option, off by default, to also reimplement itself in terms of the upstream