add an opt-in cargo feature to build intrinsics from compiler-rt source

closes #63
cc #66

Author: Jorge Aparicio

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "compiler-rt"]
+	path = compiler-rt
+	url = git://github.com/rust-lang/compiler-rt.git
+	branch = rust-llvm-2016-07-18

Cargo.toml

@@ -4,6 +4,10 @@ build = "build.rs"
 name = "rustc_builtins"
 version = "0.1.0"
 
+[build-dependencies]
+gcc = "0.3.35"
+rustc-cfg = "0.1.2"
+
 [dependencies]
 
 [dependencies.rlibc]
@@ -18,6 +22,8 @@ rand = "0.3.14"
 path = "gcc_s"
 
 [features]
+# Build the missing intrinsics from compiler-rt C source code
+c = []
 weak = ["rlibc/weak"]
 
 [workspace]