Another problem is that the backtrace facility wasn't being
enabled for non-Linux targets even if the target OS has the
backtrace functions. Also, the stacktrace functions inside
mysqltest were being used without proper checks for their
presence in the build.


client/mysqltest.c:
  Only use stacktrace functions if they are available.
configure.in:
  Check if the compiler defines __bss_start
include/my_stacktrace.h:
  Enable stacktrace if system has backtrace functions.
mysys/stacktrace.c:
  Use backtrace functions if the system supports it.
sql/mysqld.cc:
  Only use stacktrace functions if they are available.