[Makefile] Create Symbolic Link If Not Exist

Create a symbolic link to directory if the symlink does not exist in Makefile. Otherwise do nothing.

[ -L tipitaka/common ] || (cd tipitaka; ln -s ../common/ common)

Tested on Ubuntu Linux 15.10.

[1]	makefile check if symlink exists bash - How to check if symlink exists - Stack Overflow linux - Using makefile check if symlink exists and create if it doesn't - Stack Overflow Makefile target with wildcard to create symlink using target string - Stack Overflow

[2]	use Makefile for setup and development (unfinished) · siongui/pali@333e8c5 · GitHub

[3]	Package write provides a way to atomically create or replace a file or symbolic link. : golang