Move SetThreadPriority implementation to util.cpp instead of the header

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

src/util.h

@@ -342,34 +342,7 @@ bool TimingResistantEqual(const T& a, const T& b)
     return accumulator == 0;
 }
 
-#ifdef WIN32
-inline void SetThreadPriority(int nPriority)
-{
-    SetThreadPriority(GetCurrentThread(), nPriority);
-}
-#else
-
-// 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)
-
-inline void SetThreadPriority(int nPriority)
-{
-    // It's unclear if it's even possible to change thread priorities on Linux,
-    // but we really and truly need it for the generation threads.
-#ifdef PRIO_THREAD
-    setpriority(PRIO_THREAD, 0, nPriority);
-#else
-    setpriority(PRIO_PROCESS, 0, nPriority);
-#endif
-}
-#endif
-
+void SetThreadPriority(int nPriority);
 void RenameThread(const char* name);
 
 // Standard wrapper for do-something-forever thread functions.