Add support for building with static glibc

This will need corresponding changes in rust-lang/rust to activate, but
this will make it possible to make those changes.

Note that despite the apparent redundancy in config directives, the link
directives cannot be simplified any further. Attempting to factor out
the checks for `target_feature = "crt-static"` does not work.

Author: joshtriplett

src/lib.rs

@@ -19,7 +19,7 @@
 // Attributes needed when building as part of the standard library
 #![cfg_attr(
     feature = "rustc-dep-of-std",
-    feature(cfg_target_vendor, link_cfg, no_core)
+    feature(cfg_target_vendor, link_cfg, no_core, static_nobundle)
 )]
 #![cfg_attr(libc_thread_local, feature(thread_local))]
 // Enable extra lints:

src/unix/mod.rs

@@ -298,6 +298,22 @@ cfg_if! {
     } else if #[cfg(feature = "std")] {
         // cargo build, don't pull in anything extra as the libstd dep
         // already pulls in all libs.
+    } else if #[cfg(all(target_os = "linux",
+                        target_env = "gnu",
+                        feature = "rustc-dep-of-std"))] {
+        #[link(name = "c", kind = "static-nobundle",
+            cfg(target_feature = "crt-static"))]
+        #[link(name = "m", kind = "static-nobundle",
+            cfg(target_feature = "crt-static"))]
+        #[link(name = "pthread", kind = "static-nobundle",
+            cfg(target_feature = "crt-static"))]
+        #[link(name = "rt", kind = "static-nobundle",
+            cfg(target_feature = "crt-static"))]
+        #[link(name = "c", cfg(not(target_feature = "crt-static")))]
+        #[link(name = "m", cfg(not(target_feature = "crt-static")))]
+        #[link(name = "pthread", cfg(not(target_feature = "crt-static")))]
+        #[link(name = "rt", cfg(not(target_feature = "crt-static")))]
+        extern {}
     } else if #[cfg(target_env = "musl")] {
         #[cfg_attr(feature = "rustc-dep-of-std",
                    link(name = "c", kind = "static",