Move libc build def to //build_extra/rust

This is preperation for adding many more third_party crates.

BUILD.gn

@@ -14,28 +14,20 @@ config("deno_config") {
 
 rust_executable("deno") {
   source_root = "src/main.rs"
-  extern = [ ":libc" ]
+  extern = [ "$rust_build:libc" ]
   deps = [
     ":libdeno",
   ]
 }
 
-rust_component("libc") {
-  source_root = "third_party/rust_crates/libc/src/lib.rs"
-  cfg = [
-    "feature=\"default\"",
-    "feature=\"use_std\"",
-  ]
-}
-
 rust_component("handlers") {
   source_root = "src/handlers.rs"
-  extern = [ ":libc" ]
+  extern = [ "$rust_build:libc" ]
 }
 
 rust_test("handlers_test") {
   source_root = "src/handlers.rs"
-  extern = [ ":libc" ]
+  extern = [ "$rust_build:libc" ]
 }
 
 executable("deno_cc") {

build_extra/rust/BUILD.gn

@@ -13,3 +13,11 @@ rust_component("stdlib") {
     libs = [ "userenv.lib" ]
   }
 }
+
+rust_component("libc") {
+  source_root = "//third_party/rust_crates/libc/src/lib.rs"
+  cfg = [
+    "feature=\"default\"",
+    "feature=\"use_std\"",
+  ]
+}

build_extra/rust/rust.gni

@@ -1,5 +1,10 @@
 stdlib_label = "//build_extra/rust:stdlib"
 
+declare_args() {
+  # Absolute path of rust build files.
+  rust_build = "//build_extra/rust/"
+}
+
 template("run_rustc") {
   action(target_name) {
     assert(defined(invoker.source_root), "Must specify source_root")