constexpr version of abs

Source/engine.h

@@ -36,6 +36,7 @@
 #include "appfat.h"
 #include "miniwin/miniwin.h"
 #include "utils/stdcompat/cstddef.hpp"
+#include "utils/stdcompat/abs.h"
 
 #define TILE_WIDTH 64
 #define TILE_HEIGHT 32
@@ -190,7 +191,7 @@ struct Point {
 
 	constexpr friend Point abs(Point a)
 	{
-		return { std::abs(a.x), std::abs(a.y) };
+		return { abs(a.x), abs(a.y) };
 	}
 
 	constexpr int ManhattanDistance(Point other) const

Source/utils/stdcompat/abs.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <algorithm>
+
+namespace devilution {
+
+template <typename T>
+constexpr T abs(const T &a)
+{
+#if defined(__GNUC__) || defined(__GNUG__) || defined(_MSC_VER)
+	return std::abs(a);
+#else
+	return (a < 0) ? -a : a;
+#endif
+}
+
+}