Add a test to ensure no empty thread_locals are being generated

Author: FranciscoTGouveia

tests/unit/extern_global.c

@@ -0,0 +1,4 @@
+extern int x;
+int x;
+
+int main(void) { return x; }

tests/unit/out/refcount/extern_global.rs

@@ -0,0 +1,17 @@
+extern crate libcc2rs;
+use libcc2rs::*;
+use std::cell::RefCell;
+use std::collections::BTreeMap;
+use std::io::prelude::*;
+use std::io::{Read, Seek, Write};
+use std::os::fd::AsFd;
+use std::rc::{Rc, Weak};
+thread_local!(
+    pub static x_0: Value<i32> = <Value<i32>>::default();
+);
+pub fn main() {
+    std::process::exit(main_0());
+}
+fn main_0() -> i32 {
+    return (*x_0.with(Value::clone).borrow());
+}

tests/unit/out/unsafe/extern_global.rs

@@ -0,0 +1,17 @@
+extern crate libc;
+use libc::*;
+extern crate libcc2rs;
+use libcc2rs::*;
+use std::collections::BTreeMap;
+use std::io::{Read, Seek, Write};
+use std::os::fd::{AsFd, FromRawFd, IntoRawFd};
+use std::rc::Rc;
+pub static mut x_0: i32 = unsafe { 0_i32 };
+pub fn main() {
+    unsafe {
+        std::process::exit(main_0() as i32);
+    }
+}
+unsafe fn main_0() -> i32 {
+    return x_0;
+}