[autoconf] Fix MinGW build

This is the autoconf analog of r251201. I realize autoconf is
deprecated, but while it's in tree, it should at least be kept working.

Also add the deprecation message to configure.ac such that AutoRegen
actually picks ip up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254215 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed