rustc: implement -C link-arg

this flag lets you pass a _single_ argument to the linker but can be
used _repeatedly_. For example, instead of using:

```
rustc -C link-args='-l bar' (..)
```

you could write

```
rustc -C link-arg='-l' -C link-arg='bar' (..)
```

This new flag can be used with RUSTFLAGS where `-C link-args` has
problems with "nested" spaces:

```
RUSTFLAGS='-C link-args="-Tlayout.ld -nostartfiles"'
```

This passes three arguments to rustc: `-C` `link-args="-Tlayout.ld` and
`-nostartfiles"` to `rustc`. That's not what we meant. But this does
what we want:

```
RUSTFLAGS='-C link-arg=-Tlayout.ld -C link-arg=-nostartfiles`
```

cc rust-lang/rfcs#1509

src/librustc/session/config.rs

@@ -606,6 +606,7 @@ macro_rules! options {
         pub const parse_opt_bool: Option<&'static str> =
             Some("one of: `y`, `yes`, `on`, `n`, `no`, or `off`");
         pub const parse_string: Option<&'static str> = Some("a string");
+        pub const parse_string_push: Option<&'static str> = Some("a string");
         pub const parse_opt_string: Option<&'static str> = Some("a string");
         pub const parse_list: Option<&'static str> = Some("a space-separated list of strings");
         pub const parse_opt_list: Option<&'static str> = Some("a space-separated list of strings");
@@ -668,6 +669,13 @@ macro_rules! options {
             }
         }
 
+        fn parse_string_push(slot: &mut Vec<String>, v: Option<&str>) -> bool {
+            match v {
+                Some(s) => { slot.push(s.to_string()); true },
+                None => false,
+            }
+        }
+
         fn parse_list(slot: &mut Vec<String>, v: Option<&str>)
                       -> bool {
             match v {
@@ -743,6 +751,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "tool to assemble archives with"),
     linker: Option<String> = (None, parse_opt_string, [UNTRACKED],
         "system linker to link outputs with"),
+    link_arg: Vec<String> = (vec![], parse_string_push, [UNTRACKED],
+        "a single extra argument to pass to the linker (can be used several times)"),
     link_args: Option<Vec<String>> = (None, parse_opt_list, [UNTRACKED],
         "extra arguments to pass to the linker (space separated)"),
     link_dead_code: bool = (false, parse_bool, [UNTRACKED],

src/librustc_trans/back/link.rs

@@ -753,7 +753,8 @@ fn link_args(cmd: &mut Linker,
         let empty_vec = Vec::new();
         let empty_str = String::new();
         let args = sess.opts.cg.link_args.as_ref().unwrap_or(&empty_vec);
-        let mut args = args.iter().chain(used_link_args.iter());
+        let more_args = &sess.opts.cg.link_arg;
+        let mut args = args.iter().chain(more_args.iter()).chain(used_link_args.iter());
         let relocation_model = sess.opts.cg.relocation_model.as_ref()
                                    .unwrap_or(&empty_str);
         if (t.options.relocation_model == "pic" || *relocation_model == "pic")
@@ -843,6 +844,7 @@ fn link_args(cmd: &mut Linker,
     if let Some(ref args) = sess.opts.cg.link_args {
         cmd.args(args);
     }
+    cmd.args(&sess.opts.cg.link_arg);
     cmd.args(&used_link_args);
 }