Only require the target spec if the c-feature is enabled

fixes rust-lang#87

Author: oli-obk

Cargo.toml

@@ -5,7 +5,7 @@ name = "compiler_builtins"
 version = "0.1.0"
 
 [build-dependencies]
-rustc-cfg = "0.3.0"
+rustc-cfg = { version = "0.3.0", optional = true }
 
 [build-dependencies.gcc]
 optional = true
@@ -19,7 +19,7 @@ compiler-rt = { path = "compiler-rt" }
 
 [features]
 # Build the missing intrinsics from compiler-rt C source code
-c = ["gcc"]
+c = ["gcc", "rustc-cfg"]
 # Mark this crate as the #![compiler_builtins] crate
 compiler-builtins = []
 default = ["compiler-builtins"]