w32threads: Don't use function pointers when linking directly to newer APIs

This reduces the call overhead slightly. More noticeably, it
restores the earlier (unintended?) feature that condition variable
functions work just fine even if w32thread_init() hasn't been called.
This was broken as a side effect of 4622f11f, if explicitly targeting
Vista+.

This makes w32threading work in VP8 again, if targeting Vista+.
Signed-off-by: NMartin Storsjö <martin@martin.st>

compat/w32pthreads.h

@@ -61,11 +61,18 @@ typedef struct pthread_cond_t {
 } pthread_cond_t;
 
 /* function pointers to conditional variable API on windows 6.0+ kernels */
+#if _WIN32_WINNT < 0x0600
 static void (WINAPI *cond_broadcast)(pthread_cond_t *cond);
 static void (WINAPI *cond_init)(pthread_cond_t *cond);
 static void (WINAPI *cond_signal)(pthread_cond_t *cond);
 static BOOL (WINAPI *cond_wait)(pthread_cond_t *cond, pthread_mutex_t *mutex,
                                 DWORD milliseconds);
+#else
+#define cond_init      InitializeConditionVariable
+#define cond_broadcast WakeAllConditionVariable
+#define cond_signal    WakeConditionVariable
+#define cond_wait      SleepConditionVariableCS
+#endif
 
 static unsigned __stdcall attribute_align_arg win32thread_worker(void *arg)
 {
@@ -267,11 +274,6 @@ static void w32thread_init(void)
         (void*)GetProcAddress(kernel_dll, "WakeConditionVariable");
     cond_wait      =
         (void*)GetProcAddress(kernel_dll, "SleepConditionVariableCS");
-#else
-    cond_init      = InitializeConditionVariable;
-    cond_broadcast = WakeAllConditionVariable;
-    cond_signal    = WakeConditionVariable;
-    cond_wait      = SleepConditionVariableCS;
 #endif
 
 }