Replace BUILD_DEBUG by NDEBUG

Use the "standard" NDEBUG definition, which is used by assert().

app/src/main.c

@@ -625,7 +625,7 @@ main(int argc, char *argv[]) {
         return 1;
     }
 
-#ifdef BUILD_DEBUG
+#ifndef NDEBUG
     SDL_LogSetAllPriority(SDL_LOG_PRIORITY_DEBUG);
 #endif
 

app/src/tiny_xpm.c

@@ -36,7 +36,7 @@ find_color(struct index *index, int len, char c, uint32_t *color) {
 // (non-const) "char *"
 SDL_Surface *
 read_xpm(char *xpm[]) {
-#if SDL_ASSERT_LEVEL >= 2
+#ifndef NDEBUG
     // patch the XPM to change the icon color in debug mode
     xpm[2] = ".	c #CC00CC";
 #endif