gcd is not available in msvc until 2017.3 P2, back to boost::math::gcd for now

apps/launcher/graphicspage.cpp

@@ -1,5 +1,6 @@
 #include "graphicspage.hpp"
 
+#include <boost/math/common_factor.hpp>
 #include <QDesktopWidget>
 #include <QMessageBox>
 #include <QDir>
@@ -16,7 +17,7 @@
 
 QString getAspect(int x, int y)
 {
-    int gcd = std::__gcd (x, y);
+    int gcd = boost::math::gcd (x, y);
     int xaspect = x / gcd;
     int yaspect = y / gcd;
     // special case: 8 : 5 is usually referred to as 16:10

apps/openmw/mwgui/settingswindow.cpp

@@ -8,6 +8,7 @@
 #include <MyGUI_TabControl.h>
 
 #include <boost/algorithm/string.hpp>
+#include <boost/math/common_factor.hpp>
 
 #include <SDL_video.h>
 
@@ -54,7 +55,7 @@ namespace
 
     std::string getAspect (int x, int y)
     {
-        int gcd = std::__gcd (x, y);
+        int gcd = boost::math::gcd (x, y);
         int xaspect = x / gcd;
         int yaspect = y / gcd;
         // special case: 8 : 5 is usually referred to as 16:10