Auto merge of #2100 - TheDoctor314:gettid, r=JohnTitor

Add gettid() for Linux

Fixes #2076

Author: bors

libc-test/build.rs

@@ -2737,6 +2737,9 @@ fn test_linux(target: &str) {
             // assume it's a int instead.
             "getnameinfo" if uclibc => true,
 
+            // FIXME: This needs musl 1.2.2 or later.
+            "gettid" if musl => true,
+
             _ => false,
         }
     });

src/unix/linux_like/mod.rs

@@ -1666,6 +1666,7 @@ extern "C" {
         flags: ::c_int,
     ) -> ::ssize_t;
     pub fn uname(buf: *mut ::utsname) -> ::c_int;
+    pub fn gettid() -> ::pid_t;
 }
 
 cfg_if! {