Improve configure.ac and changes to USE_USER_EVENT_LOGGING.

Changes to configure.ac:
 Move AC_USE_SYSTEM_EXTENSIONS near the top to prevent autoconf warnings. 
Only warn for unset SAC2BASE when building sac4snet.
Add option --disable-setaffinity to enable clean speedup measurements with an increasing number of cores.

Changes to USE_USER_EVENT_LOGGING:
Disable USE_USER_EVENT_LOGGING by default in the source file.
This is better enabled via configure, compile_cmd or Makefile.
Also, when USE_USER_EVENT_LOGGING is unset then don't do any
event logging activities at all, such as opening log files,
or initializing event logging threads.