make the gcc crate optional

it's only required when the "c" feature is enabled

Cargo.toml

@@ -6,7 +6,10 @@ version = "0.1.0"
 
 [build-dependencies]
 rustc-cfg = "0.3.0"
-gcc = "0.3.36"
+
+[build-dependencies.gcc]
+optional = true
+version = "0.3.36"
 
 [dev-dependencies]
 quickcheck = "0.3.1"
@@ -16,7 +19,7 @@ compiler-rt = { path = "compiler-rt" }
 
 [features]
 # Build the missing intrinsics from compiler-rt C source code
-c = []
+c = ["gcc"]
 # Mark this crate as the #![compiler_builtins] crate
 compiler-builtins = []
 default = ["compiler-builtins"]

build.rs

@@ -1,18 +1,23 @@
+#[cfg(feature = "c")]
 extern crate gcc;
 extern crate rustc_cfg;
 
+#[cfg(feature = "c")]
 use std::collections::BTreeMap;
 use std::io::Write;
+#[cfg(feature = "c")]
 use std::path::Path;
 use std::{env, io, process};
 
 use rustc_cfg::Cfg;
 
+#[cfg(feature = "c")]
 struct Sources {
     // SYMBOL -> PATH TO SOURCE
     map: BTreeMap<&'static str, &'static str>,
 }
 
+#[cfg(feature = "c")]
 impl Sources {
     fn new() -> Sources {
         Sources { map: BTreeMap::new() }
@@ -65,10 +70,12 @@ fn main() {
     // target triple. This is usually correct for our built-in targets but can break in presence of
     // custom targets, which can have arbitrary names.
     let llvm_target = target.split('-').collect::<Vec<_>>();
-    let target_vendor = target_vendor.as_ref().unwrap();
 
     // Build missing intrinsics from compiler-rt C source code
-    if env::var_os("CARGO_FEATURE_C").is_some() {
+    match () {
+        #[cfg(feature = "c")]
+        () => {
+            let target_vendor = target_vendor.as_ref().unwrap();
             let cfg = &mut gcc::Config::new();
 
             if target_env == "msvc" {
@@ -406,6 +413,9 @@ fn main() {
 
             cfg.compile("libcompiler-rt.a");
         }
+        #[cfg(not(feature = "c"))]
+        () => {}
+    }
 
     // To filter away some flaky test (see src/float/add.rs for details)
     if llvm_target[0].starts_with("arm") &&