doc: update configure ar doc to mention libtool 2.4.7

Author: fanquake

configure.ac

@@ -31,6 +31,7 @@ BITCOIN_MP_NODE_NAME=bitcoin-node
 BITCOIN_MP_GUI_NAME=bitcoin-gui
 
 dnl Unless the user specified ARFLAGS, force it to be cr
+dnl This is also the default as-of libtool 2.4.7
 AC_ARG_VAR([ARFLAGS], [Flags for the archiver, defaults to <cr> if not set])
 if test "${ARFLAGS+set}" != "set"; then
   ARFLAGS="cr"