Makefile: Blacklist functions that threaded programs cannot use safely.

Makefile: Blacklist functions that threaded programs cannot use safely.

Some functions that POSIX says cannot be used safely in multithreaded
programs are not on the initial blacklist:

    - getenv() should be safe in real implementations in the absence of
      changes to the environment.  (putenv() and setenv() are blacklisted.)

    - We only use getopt() before spawning extra threads, and I expect this
      to continue to be true.

Signed-off-by: Ben Pfaff <blp@nicira.com>