Save Reid's notes from bugzilla on the proper procedure to upgrade libtool

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16947 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/autoconf/README.TXT b/autoconf/README.TXT
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+Upgrading Libtool
+-----------------
+
+If you are in the mood to upgrade libtool, you must do the following:
+
+1. configure/build/install libtool with --prefix=<PFX>
+2. Copy <PFX>/share/aclocal/libtool.m4 to llvm/autoconf/m4/libtool.m4
+3. Copy <PFX>/share/libtool/ltmain.sh to llvm/autoconf/ltmain.sh
+4. Adjust the llvm/autoconf/m4/libtool.m4 file to ensure that the variable
+   default_ofile is set to "mklib" instead of "libtool"
+5. Possibly adjust llvm/autoconf/m4/libtool.m4 to get rid of autoconf 
+   warnings related to obsolete autconf features (e.g. AC_TRY_LINK)