Debug mode option to error on evthread init AFTER other event calls.

- A handy event_enable_debug_mode() feature which will error and abort the
  application if any thread-aware libevent functions are called BEFORE the
  evthread API has been initialized (manually, or through
  evthread_use_windows_threads() / evthread_use_pthreads()

- This is done by setting the global debug variable
  'event_debug_created_threadable_ctx_' whenever the following functions
  are called:

     evthreadimpl_lock_alloc_()
     evthreadimpl_cond_alloc_()
     event_base_new_with_config() <- this checks to see if the thread
                                     callbacks are enabled first, so we
                                     have to manually set the variable.

- Example:

int main(int argc, char ** argv) {
    struct event_base * base;

    event_enable_debug_mode();

    base = event_base_new();

    evthread_use_pthreads();

    return 0;
}

When executed, the program will throw an error and exit:

[err] evthread initialization must be called BEFORE anything else!

1 files changed, 16 insertions, 0 deletions

diff --git a/event.c b/event.c
index acef2b24..a932c3bb 100644
--- a/event.c
+++ b/event.c
@@ -199,6 +199,20 @@ eq_debug_entry(const struct event_debug_entry *a,
 }
 
 int event_debug_mode_on_ = 0;
+
+
+/**
+ * @brief debug mode variable which is set for any function/structure that needs
+ *        to be shared across threads (if thread support is enabled).
+ *
+ *        When and if evthreads are initialized, this variable will be evaluated,
+ *        and if set to something other than zero, this means the evthread setup 
+ *        functions were called out of order.
+ *
+ *        See: "Locks and threading" in the documentation.
+ */
+int event_debug_created_threadable_ctx_ = 0;
+
 /* Set if it's too late to enable event_debug_mode. */
 static int event_debug_mode_too_late = 0;
 #ifndef EVENT__DISABLE_THREAD_SUPPORT
@@ -656,6 +670,8 @@ event_base_new_with_config(const struct event_config *cfg)
 	/* prepare for threading */
 
 #ifndef EVENT__DISABLE_THREAD_SUPPORT
+	event_debug_created_threadable_ctx_ = 1;
+
 	if (EVTHREAD_LOCKING_ENABLED() &&
 	    (!cfg || !(cfg->flags & EVENT_BASE_FLAG_NOLOCK))) {
 		int r;