add a lib for rendering text in sdl2 and rules to make in makefile

SDL2_ttf/external/download.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+set -e
+
+ARGUMENTS="$*"
+
+cd $(dirname "$0")/..
+cat .gitmodules | \
+while true; do
+    read module || break
+    read line; set -- $line
+    path=$3
+    read line; set -- $line
+    url=$3
+    read line; set -- $line
+    branch=$3
+
+    if [ -z "$ARGUMENTS" ]; then
+        ARGUMENTS="--depth 1"
+    fi
+
+    git clone --filter=blob:none $url $path -b $branch --recursive $ARGUMENTS
+done