+ add option to switch off precompiled header

src/Base/Quantity.cpp

@@ -23,9 +23,12 @@
 #include "PreCompiled.h"
 #ifndef _PreComp_
 # include <sstream>
+# ifdef FC_OS_WIN32
+# define _USE_MATH_DEFINES
+# endif // FC_OS_WIN32
+# include <cmath>
 #endif
 
-#include <cmath>
 #include "Quantity.h"
 #include "Exception.h"
 #include "UnitsApi.h"