message: Fix configure error on native Windows MinGW build
diff: @@ -170,7 +170,7 @@ if test "x$want_boost" = "xyes"; then
  AC_MSG_RESULT(yes)
  succeeded=yes
  found_system=yes
  - ],[
  + ],[:
  ])
  AC_LANG_POP([C++])
  @@ -263,7 +263,7 @@ if test "x$want_boost" = "xyes"; then
  AC_MSG_RESULT(yes)
  succeeded=yes
  found_system=yes
  - ],[
  + ],[:
  ])
  AC_LANG_POP([C++])
  fi
  