Mark more symbols as static

[Yuri Gribov]

Hi all,

It seems few more symbols in make can be marked as static:
  default_load_average (src/main.o)
  directory_before_chdir (src/main.o)
  job_counter (src/job.o)
  make_sync (src/main.o)
  output_sync_option (src/main.o)
  sync_mutex (src/main.o)
I do not have copyright assignment so am not sending a proper patch
but I can go for it if this change is considered useful.

The issue has been found when playing with pet project
https://github.com/yugr/Localizer

-- 
Best regards,
Yuri