Convert non-mandatory attributes to macros

They're shorter and more portable.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/kernel.h

@@ -2,18 +2,19 @@
 // Copyright (c) Yuxuan Shui <yshuiv7@gmail.com>
 
 #pragma once
+#include "compiler.h"
 
 /// Code for generating convolution kernels
 
 typedef struct conv {
-  int size;
-  double data[];
+	int size;
+	double data[];
 } conv;
 
 /// Calculate the sum of a rectangle part of the convolution kernel
 /// the rectangle is defined by top left (x, y), and a size (width x height)
-double
-sum_kernel(conv *map, int x, int y, int width, int height);
+double attr_const attr_pure sum_kernel(const conv *map, int x, int y, int width,
+                                       int height);
 
 /// Create a kernel with gaussian distribution of radius r
 conv *gaussian_kernel(double r);