Makes external/git/clone.sh POSIX shell compatible (letoram#261)

Author: cipharius

external/git/clone.sh

@@ -1,14 +1,22 @@
-#!/bin/bash
+#!/bin/sh
 
 # solution plucked from http://stackoverflow.com/questions/59895
-SOURCE="${BASH_SOURCE[0]}"
+SOURCE="$0"
 while [ -h "$SOURCE" ]; do
   DIR="$( cd -P "$( dirname "$SOURCE" )" && pwd )"
   SOURCE="$(readlink "$SOURCE")"
-  [[ $SOURCE != /* ]] && SOURCE="$DIR/$SOURCE"
+  case $SOURCE in
+    "/"*) true ;;
+    *) SOURCE="$DIR/$SOURCE" ;;
+  esac
 done
 DIR="$( cd -P "$( dirname "$SOURCE" )" && pwd )"
 
+if [ ! -f "$DIR/clone.sh" ] || [ -h "$DIR/clone.sh" ]; then
+  >&2 echo 'Could not resolve source directory, make sure to execute the source file instead of sourcing it'
+  exit 1
+fi
+
 git clone http://luajit.org/git/luajit-2.0.git "$DIR/luajit"
 git clone https://github.com/letoram/openal.git "$DIR/openal"
 git clone https://github.com/libuvc/libuvc.git "$DIR/libuvc"