fix some errors and warnings for GCC 6

* do not use std::abs() because math.h includes one, that converts ints
  to double. Use own iabs(), which directly uses labs() instead.
* don't use the word "gamma" as name for variables
* don't use a homebrew round() function

source/src/worldocull.cpp

@@ -72,7 +72,7 @@ void computeraytable(float vx, float vy, float fov)
                 {
                     float dist = (float)(fabs(sx-vx)+fabs(sy-vy));
                     if(firstquarter || dist > rdist[k]) rdist[k] = dist;
-                    if(firstquarter) ndist[k] = max(abs(int(sx-vx)), abs(int(sy-vy)));
+                    if(firstquarter) ndist[k] = max(iabs(sx-vx), iabs(sy-vy));
                     break;
                 }
             }