Remove autoconf support

Summary:
This patch is provided in preparation for removing autoconf on 1/26. The proposal to remove autoconf on 1/26 was discussed on the llvm-dev thread here: http://lists.llvm.org/pipermail/llvm-dev/2016-January/093875.html

"Now I am become Death, the destroyer of worlds."
-J. Robert Oppenheimer

Reviewers: chandlerc, grosbach, bob.wilson, echristo

Subscribers: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D16475

llvm-svn: 258864