Apply #![feature(proc_macro_hygiene)] when needed

This allows the rust-bpf-builder toolchain to build the sdk

sdk/build.rs

@@ -17,6 +17,11 @@ fn main() {
         }
         Channel::Dev => {
             println!("cargo:rustc-cfg=RUSTC_WITH_SPECIALIZATION");
+            // See https://github.com/solana-labs/solana/issues/11055
+            // We may be running the custom `rust-bpf-builder` toolchain,
+            // which currently needs `#![feature(proc_macro_hygiene)]` to
+            // be applied.
+            println!("cargo:rustc-cfg=RUSTC_NEEDS_PROC_MACRO_HYGIENE");
         }
     }
 }

sdk/src/lib.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(RUSTC_WITH_SPECIALIZATION, feature(specialization))]
+#![cfg_attr(RUSTC_NEEDS_PROC_MACRO_HYGIENE, feature(proc_macro_hygiene))]
 
 // Allows macro expansion of `use ::solana_sdk::*` to work within this crate
 extern crate self as solana_sdk;