Move SetThreadPriority implementation to util.cpp instead of the header

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

src/util.cpp

@@ -1286,3 +1286,16 @@ std::string FormatParagraph(const std::string in, size_t width, size_t indent)
     }
     return out.str();
 }
+
+void SetThreadPriority(int nPriority)
+{
+#ifdef WIN32
+    SetThreadPriority(GetCurrentThread(), nPriority);
+#else // WIN32
+#ifdef PRIO_THREAD
+    setpriority(PRIO_THREAD, 0, nPriority);
+#else // PRIO_THREAD
+    setpriority(PRIO_PROCESS, 0, nPriority);
+#endif // PRIO_THREAD
+#endif // WIN32
+}