Add src/cmath.h for <math.h> portability between platforms

src/cmath.h

@@ -0,0 +1,17 @@
+// Provide an uniform interface for basic math functions between AVR libc and newer
+// standards that support <cmath>
+#pragma once
+
+#ifndef __AVR__
+    #include <cmath>
+#else
+
+    // AVR libc doesn't support cmath
+    #include <math.h>
+
+    // Use builtin functions for min/max/abs
+    #define min(a, b) __builtin_min((a, b))
+    #define max(a, b) __builtin_max((a, b))
+    #define abs(n) __builtin_abs((n))
+
+#endif