Deprecate in-source autotools builds

This is a followup from:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150323/268067.html

Upgrade instructions:
  $ mv llvm/include/llvm/Config/config.h ./config.h.BACKUP
  # copy the configure line from line 7 of llvm/config.log
  # (for example: `$ ./configure --no-create --no-recursion`)
  $ mkdir build
  $ cd build
  # run the configure line, but this time with '../llvm' at the beginning:
  $ ../llvm/configure --no-create --no-recursion

These warnings will soon be turned into hard errors after a week.  Speak up now
if this is going to be a problem for you.

llvm-svn: 236399