Move SetThreadPriority implementation to util.cpp instead of the header

Put the THREAD_* and PRIO_ constants in compat.h.

src/compat.h

@@ -59,4 +59,15 @@ typedef u_int SOCKET;
 #define SOCKET_ERROR        -1
 #endif
 
+#ifndef WIN32
+// PRIO_MAX is not defined on Solaris
+#ifndef PRIO_MAX
+#define PRIO_MAX 20
+#endif
+#define THREAD_PRIORITY_LOWEST          PRIO_MAX
+#define THREAD_PRIORITY_BELOW_NORMAL    2
+#define THREAD_PRIORITY_NORMAL          0
+#define THREAD_PRIORITY_ABOVE_NORMAL    (-2)
+#endif
+
 #endif // _BITCOIN_COMPAT_H