Fix bug#27520. For some unknown reason, libtool.m4 attempts

to find "ar" but if it cannot be found sets AR=false.  This
leads to confusing failures during the build rather than at
configure time.

We have our own checks for ar, but as AR was already set
earlier by the libtool tests they were never exectuted.

Therefore, update the tests so that we catch any libtool
failures, and run AC_CHECK_PROG explicitly to ensure that we
see sensible output from configure prior to any potential
failure.


config/ac-macros/misc.m4:
  AC_CHECK_PROG does nothing if the first argument (variable) is
  already set, so remove redundant check for the empty string.
  This also ensures that a 'checking for ar' message is printed
  prior to any failure, which is far less confusing.

config/ac-macros/misc.m4

@@ -773,11 +773,8 @@ case $CXX_VERSION in
     ARFLAGS="-xar -o"
   ;;
   *)
-    if test -z "$AR"
-    then
     AC_CHECK_PROG([AR], [ar], [ar])
-    fi
-    if test -z "$AR"
+    if test -z "$AR" || test "$AR" = "false"
     then
       AC_MSG_ERROR([You need ar to build the library])
     fi