Add code conditionally for older macOS SDKs.

Frameworks/SSEQPlayer/SSEQPlayer/Channel.cpp

@@ -52,6 +52,29 @@ double Channel::window_lut[Channel::SINC_SAMPLES + 1];
 static const double M_PI = 3.14159265358979323846;
 #endif
 
+#if !defined(MAC_OS_X_VERSION_10_13) || (MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_13)
+// Code from http://hbfs.wordpress.com/2008/08/05/branchless-equivalents-of-simple-functions/
+inline int32_t sex(int32_t x)
+{
+    union
+    {
+        // let us suppose long is twice as wide as int
+        int64_t w;
+        
+        // should be hi,lo on a big endian machine
+        struct { int32_t lo, hi; } s;
+    } z;
+    z.w = x;
+    
+    return z.s.hi;
+}
+
+inline uint32_t abs(int32_t x)
+{
+    return (x ^ sex(x)) - sex(x);
+}
+#endif
+
 // Code from http://learningcppisfun.blogspot.com/2010/04/comparing-floating-point-numbers.html
 template<typename T> inline bool fEqual(T x, T y, int N = 1)
 {