Auto merge of #183 - servo:build-fix, r=jdm

Cherry-pick the fix for bug 1533969.

Otherwise I can't build :)

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/mozjs/183)
<!-- Reviewable:end -->

Author: bors-servo

Cargo.toml

@@ -2,7 +2,7 @@
 name = "mozjs_sys"
 description = "System crate for the Mozilla SpiderMonkey JavaScript engine."
 repository = "https://github.com/servo/mozjs/"
-version = "0.66.0"
+version = "0.66.1"
 authors = ["Mozilla"]
 links = "mozjs"
 build = "build.rs"

mozjs/js/src/util/NativeStack.cpp

@@ -25,7 +25,7 @@
 #    include <sys/syscall.h>
 #    include <sys/types.h>
 #    include <unistd.h>
-static pid_t gettid() { return syscall(__NR_gettid); }
+#    define gettid() static_cast<pid_t>(syscall(__NR_gettid))
 #  endif
 #else
 #  error "Unsupported platform"