Add test for deduplicate native lib in linking

src/test/run-make-fulldeps/link-dedup/Makefile

@@ -0,0 +1,12 @@
+# ignore-msvc
+
+-include ../tools.mk
+
+all:
+	$(RUSTC) depa.rs
+	$(RUSTC) depb.rs
+	$(RUSTC) depc.rs
+	$(RUSTC) empty.rs --cfg bar 2>&1 | $(CGREP) '"-ltesta" "-ltestb" "-ltesta"'
+	$(RUSTC) empty.rs 2>&1 | $(CGREP) '"-ltesta"'
+	$(RUSTC) empty.rs 2>&1 | $(CGREP) -v '"-ltestb"'
+	$(RUSTC) empty.rs 2>&1 | $(CGREP) -v '"-ltesta" "-ltesta"'

src/test/run-make-fulldeps/link-dedup/depa.rs

@@ -0,0 +1,7 @@
+#![crate_type = "rlib"]
+
+#[link(name = "testa")]
+extern "C" {}
+
+#[link(name = "testa")]
+extern "C" {}

src/test/run-make-fulldeps/link-dedup/depb.rs

@@ -0,0 +1,8 @@
+#![feature(link_cfg)]
+#![crate_type = "rlib"]
+
+#[link(name = "testb", cfg(foo))]
+extern "C" {}
+
+#[link(name = "testb", cfg(bar))]
+extern "C" {}

src/test/run-make-fulldeps/link-dedup/depc.rs

@@ -0,0 +1,4 @@
+#![crate_type = "rlib"]
+
+#[link(name = "testa")]
+extern "C" {}

src/test/run-make-fulldeps/link-dedup/empty.rs

@@ -0,0 +1,5 @@
+extern crate depa;
+extern crate depb;
+extern crate depc;
+
+fn main() {}