Depending on the windows target version (_WIN32_WINNT), the used
SDK and the build configuration the linker will see multiple
libraries exporting the same symbols.
To prevent ugly hacks (e.g. modifying cmake's default system
libraries or fragile library linking order chains) we prefix
these functions with "winpr_" and create corresponding defines
to keep the current api names.
Let the compiler know that we're comparing a volatile value.
Otherwise the compiler might nuke the comparison operation
and produce code that will spin endlessly.
MSDN documentation says it is ensured that all threads in the barrier
have finished using it before allowing the barrier to be released in
DeleteSynchronizationBarrier(). The winpr re-implementation wasn't
keeping to that requirement, which was causing occasional crashes
when shadow client tried to access already freed barrier structure.
The crash was occuring in winpr_Handle_cleanup() after finished
waiting on a barrier's event.
