docs: Fix footnote after r260042.

r260042 removed a footnote referring to autoconf, but it left
around one item still referring to that footnote (libtool), and
it didn't renumber the later footnote reference.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264663 91177308-0d34-0410-b5e6-96231b3b80d8