Sidestep link error from rustfix'ed code by using a *defined* static.

[pnkfelix]

As a drive-by, added -g to the compile-flags so that the test more
reliably fails to compile when the extern static in question is not
provided. (I.e. this is making the test more robust in the face of
potential future revisions.)

Fix #54388.

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

@bors: r+ rollup

[bors]

📌 Commit e7f8c0d has been approved by alexcrichton

[kennytm]

@bors r-

Failed in the rollup #55746 (comment) for wasm32-unknown. Is libc::c_int defined in wasm32?

Error message

---- [ui] ui/extern/extern-const.rs stdout ----
diff of stderr:

4	LL |     const rust_dbg_static_mut: libc::c_int; //~ ERROR extern items cannot be `const`
5	   |     ^^^^^ help: try using a static value: `static`
6	
-	error: aborting due to previous error
+	error[E0412]: cannot find type `c_int` in module `libc`
+	  --> $DIR/extern-const.rs:15:38
+	   |
+	LL |     const rust_dbg_static_mut: libc::c_int; //~ ERROR extern items cannot be `const`
+	   |                                      ^^^^^ not found in `libc`
+	help: possible candidate is found in another module, you can import it into scope
+	   |
+	LL | use std::os::raw::c_int;
+	   |
8	
+	error: aborting due to 2 previous errors
+	
+	For more information about this error, try `rustc --explain E0412`.
9	


The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/extern/extern-const/extern-const.stderr
diff of fixed:

10	#![feature(libc)]
11	extern crate libc;
12	
+	use std::os::raw::c_int;
+	
13	#[link(name = "rust_test_helpers", kind = "static")]
14	extern "C" {
15	    static rust_dbg_static_mut: libc::c_int; //~ ERROR extern items cannot be `const`


The actual fixed differed from the expected fixed.

[pnkfelix]

Is libc::c_int defined in wasm32?

Probably not. But I'm pretty sure extern { static ... } doesn't make any sense there either.

So I think I'll just add a // ignore-wasm32 to the test in question.

[pnkfelix]

(marking as Work-In-Progress while dafd2e8 gets tested...)

[pnkfelix]

@bors r=alexcrichton rollup

[bors]

📌 Commit 34ffbdb has been approved by alexcrichton