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

    "This is the way [autoconf] ends
    Not with a bang but a whimper."
    -T.S. Eliot

Reviewers: chandlerc, grosbach, bob.wilson, echristo

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D16472

llvm-svn: 258862