bootstrap: Add directives to not double-link libs

Have all Cargo-built crates pass `--cfg cargobuild` and then add appropriate
`#[cfg]` definitions to all crates to avoid linking anything if this is passed.
This should help allow libstd to compile with both the makefiles and with Cargo.

src/libstd/build.rs

@@ -19,6 +19,8 @@ use std::process::Command;
 use build_helper::run;
 
 fn main() {
+    println!("cargo:rustc-cfg=cargobuild");
+
     let target = env::var("TARGET").unwrap();
     let host = env::var("HOST").unwrap();
     if !target.contains("apple") && !target.contains("msvc") {

src/libstd/rand/os.rs

@@ -269,7 +269,10 @@ mod imp {
     const kSecRandomDefault: *const SecRandom = ptr::null();
 
     #[link(name = "Security", kind = "framework")]
-    extern "C" {
+    #[cfg(not(cargobuild))]
+    extern {}
+
+    extern {
         fn SecRandomCopyBytes(rnd: *const SecRandom,
                               count: size_t, bytes: *mut u8) -> c_int;
     }

src/libstd/rtdeps.rs

@@ -12,6 +12,8 @@
 //! the standard library This varies per-platform, but these libraries are
 //! necessary for running libstd.
 
+#![cfg(not(cargobuild))]
+
 // LLVM implements the `frem` instruction as a call to `fmod`, which lives in
 // libm. Hence, we must explicitly link to it.
 //

src/libstd/sys/common/gnu/libbacktrace.rs

@@ -40,7 +40,7 @@ pub fn print(w: &mut Write, idx: isize, addr: *mut libc::c_void,
                       errnum: libc::c_int);
     enum backtrace_state {}
     #[link(name = "backtrace", kind = "static")]
-    #[cfg(not(test))]
+    #[cfg(all(not(test), not(cargobuild)))]
     extern {}
 
     extern {

src/libstd/sys/common/unwind/gcc.rs

@@ -252,6 +252,9 @@ pub mod eh_frame_registry {
     // See also: rtbegin.rs, `unwind` module.
 
     #[link(name = "gcc_eh")]
+    #[cfg(not(cargobuild))]
+    extern {}
+
     extern {
         fn __register_frame_info(eh_frame_begin: *const u8, object: *mut u8);
         fn __deregister_frame_info(eh_frame_begin: *const u8, object: *mut u8);

src/libstd/sys/unix/os.rs

@@ -339,7 +339,6 @@ pub fn args() -> Args {
 pub fn args() -> Args {
     use mem;
 
-    #[link(name = "objc")]
     extern {
         fn sel_registerName(name: *const libc::c_uchar) -> Sel;
         fn objc_msgSend(obj: NsId, sel: Sel, ...) -> NsId;
@@ -347,6 +346,8 @@ pub fn args() -> Args {
     }
 
     #[link(name = "Foundation", kind = "framework")]
+    #[link(name = "objc")]
+    #[cfg(not(cargobuild))]
     extern {}
 
     type Sel = *const libc::c_void;

src/libstd/sys/windows/c.rs

@@ -966,6 +966,9 @@ pub enum EXCEPTION_DISPOSITION {
 #[link(name = "userenv")]
 #[link(name = "shell32")]
 #[link(name = "advapi32")]
+#[cfg(not(cargobuild))]
+extern {}
+
 extern "system" {
     pub fn WSAStartup(wVersionRequested: WORD,
                       lpWSAData: LPWSADATA) -> c_int;