fix builds on macOS Catalina (10.15.7)

The `_DARWIN_BETTER_REALPATH` flag fixes the file device reading directories.

The colors change fixes an integer size error in the code below it.

I have a few other quality of life commits on the `macbuild` branch of my
git.sr.ht/~apiarian/uxn fork for more Mac convenience, but the probably don't
belong in the main branch of uxn. Maybe we can explore adding them as build
flags?

- al

build.sh

@@ -64,7 +64,7 @@ MSYS_NT*|MINGW*) # MSYS2 on Windows
 	fi
 	;;
 Darwin) # macOS
-	CFLAGS="${CFLAGS} -Wno-typedef-redefinition -D_C99_SOURCE"
+	CFLAGS="${CFLAGS} -Wno-typedef-redefinition -D_C99_SOURCE -D_DARWIN_BETTER_REALPATH"
 	UXNEMU_LDFLAGS="$(sdl2-config --cflags --static-libs)"
 	;;
 Linux|*)

src/devices/screen.c

@@ -50,7 +50,8 @@ screen_change(int x1, int y1, int x2, int y2)
 void
 screen_palette(void)
 {
-	int i, shift, colors[4];
+	int i, shift;
+	unsigned long colors[4];
 	for(i = 0, shift = 4; i < 4; ++i, shift ^= 4) {
 		Uint8
 			r = (uxn.dev[0x8 + i / 2] >> shift) & 0xf,