implement pthread_condattr_* interfaces

note that, while the attributes are stored, they are not used in
pthread_cond_init yet.

src/thread/pthread_condattr_setclock.c

@@ -0,0 +1,9 @@
+#include "pthread_impl.h"
+
+int pthread_condattr_setclock(pthread_condattr_t *a, clockid_t clk)
+{
+	if (clk < 0) return EINVAL;
+	*a &= 0x80000000;
+	*a |= clk;
+	return 0;
+}