build: remove unnecessary flag from CMAKE_FLAGS

We now append the deps install area into CMAKE_PREFIX_PATH.
vim-patch:7.4.272

Problem:  Using just "$" does not cause an error message.
Solution: Check for empty environment variable name.
          (Christian Brabandt)

https://code.google.com/p/vim/source/detail?r=00228400629e28384f7f52556c3c119ba0d0a44d