explicitly control compiler_builts/c feature from libstd

Author: RalfJung

src/libstd/Cargo.toml

@@ -18,7 +18,7 @@ panic_unwind = { path = "../libpanic_unwind", optional = true }
 panic_abort = { path = "../libpanic_abort" }
 core = { path = "../libcore" }
 libc = { path = "../rustc/libc_shim" }
-compiler_builtins = { path = "../rustc/compiler_builtins_shim" }
+compiler_builtins = { path = "../rustc/compiler_builtins_shim", features = ["c"] }
 profiler_builtins = { path = "../libprofiler_builtins", optional = true }
 unwind = { path = "../libunwind" }
 

src/rustc/compiler_builtins_shim/Cargo.toml

@@ -34,7 +34,7 @@ cc = "1.0.1"
 
 [features]
 c = []
-default = ["c", "rustbuild", "compiler-builtins"]
+default = ["rustbuild", "compiler-builtins"]
 mem = []
 rustbuild = []
 compiler-builtins = []