Fix exit code.

Index: ConfigureChecks.cmake
--- ConfigureChecks.cmake.orig
+++ ConfigureChecks.cmake
@@ -306,7 +306,7 @@ check_c_source_compiles("
 __thread int tls;
 
 int main(void) {
-    return 0;
+    return tls;
 }" HAVE_GCC_THREAD_LOCAL_STORAGE)
 
 check_c_source_compiles("