w32threads: Use newer thread synchronization functions when targeting Vista

When explicitly targeting Vista or newer (which only happens if the
caller explicitly sets _WIN32_WINNT to a high enough value via the
extra cflags option - otherwise configure script sets
-D_WIN32_WINNT=0x0502), we already unconditionally link to the
ConditionVariable functions, since 4622f11f9.

Similarly use the newer -Ex versions of CreateEvent, CreateSemaphore,
InitializeCriticalSection and WaitForSingleObject, that all appeared
in Vista. When building Windows Store applications, the older versions
of these functions aren't available, only the -Ex functions. When
doing such a build, the user can set -D_WIN32_WINNT=0x0600 to
forcibly use the newer functions instead.

Signed-off-by: Martin Storsjö <martin@martin.st>