bug#30106: [PATCH 2/2] Fix module support if threads are disabled (Bug#30106)

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Cc: Philipp Stephani <phst@google.com>
> Date: Wed, 17 Jan 2018 23:28:46 +0100
> 
> * src/systhread.c (sys_thread_equal): New function.
> * src/emacs-module.c (in_current_thread): Use it.

I'd prefer that the only file that calls systhread.c functions is
thread.c; systhread.c is supposed to be low-level code concealed from
application levels.  So this would call for another level of
indirection: add a new function to thread.c, and call that from
emacs-module.c.

Otherwise, LGTM for master; thanks.