Windows build fixes

app/app.pro

@@ -6,6 +6,7 @@
 
 QT += core gui multimedia opengl
 greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
+QMAKE_CXXFLAGS += -std=c++11
 
 TEMPLATE = app
 TARGET = sdrangel

plugins/channelrx/demodbfm/rdsdemod.cpp

@@ -22,6 +22,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+#undef M_PI
+#define M_PI 3.14159265358979323846
+#undef M_PI_2
+#define M_PI_2 1.57079632679489661923
 
 const Real RDSDemod::m_pllBeta = 50;
 const Real RDSDemod::m_fsc = 1187.5;