suppress bu_log from multiple threads simultaneously

[kennethsdavis]

There is a semaphore global to log.c that is intended to protect calls to bu_hook_call (via log_call_hooks). Because log_call_hooks can only be called if the log_hook_list is non-empty this semaphore only gets initialized when bu_log_add_hook is called the first time.

I considered the FIXME note at the top of log.c as a strong candidate for implementation from a design standpoint, but favored this solution because its development cost and risk are both low.

This pull request satisfies #100

[kennethsdavis]

Testing was performed against 7.36.0 because main HEAD won't build brlcad.dll at the time of this comment (see #105). The sample application on #100 executes as expected after applying this change.