std: Add a helper for symbols that may not exist

Right now we only attempt to call one symbol which my not exist everywhere,
__pthread_get_minstack, but this pattern will come up more often as we start to
bind newer functionality of systems like Linux.

Take a similar strategy as the Windows implementation where we use `dlopen` to
lookup whether a symbol exists or not.

src/libstd/sys/unix/mod.rs

@@ -27,6 +27,9 @@ use ops::Neg;
 #[cfg(target_os = "openbsd")]   pub use os::openbsd as platform;
 #[cfg(target_os = "solaris")]   pub use os::solaris as platform;
 
+#[macro_use]
+pub mod weak;
+
 pub mod backtrace;
 pub mod condvar;
 pub mod ext;