Only warn once on windows for unsupported timer usage

include/ACGL/Utils/PerformanceTimer.hh

@@ -139,7 +139,8 @@ class CSystemTimer : public CProcessTimer
 {
 public:
     CSystemTimer() : CProcessTimer() {
-        ACGL::Utils::error() << "No system timer present on this OS - fallback to process time" << std::endl;
+		static bool warned = false;
+		if ( !warned++ ) ACGL::Utils::error() << "No system timer present on this OS - fallback to process time" << std::endl;
     }
 };
 
@@ -147,8 +148,9 @@ public:
 class CThreadTimer : public CProcessTimer
 {
 public:
-    CThreadTimer() : CProcessTimer() {
-        ACGL::Utils::error() << "No thread timer present on this OS - fallback to process time" << std::endl;
+	CThreadTimer() : CProcessTimer() {
+		static bool warned = false;
+		if ( !warned++ ) ACGL::Utils::error() << "No thread timer present on this OS - fallback to process time" << std::endl;
     }
 };