As documented in the bug report, the double checked locking
pattern has inherent issues, and cannot guarantee correct
initialization.

This patch replaces the logic in init_available_charsets()
with the use of pthread_once(3). A wrapper function,
my_pthread_once(), is introduced and is used in lieu of direct
calls to init_available_charsets(). Related defines
MY_PTHREAD_ONCE_* are also introduced.

For the Windows platform, the implementation in lp:sysbench is
ported. For single-thread use, a simple define calls the
function and sets the pthread_once control variable.

Charset initialization is modified to use my_pthread_once().