message: Update boost system m4 macros to latest upstream
diff: # ===========================================================================
  -# http://www.gnu.org/software/autoconf-archive/ax_boost_system.html
  +# https://www.gnu.org/software/autoconf-archive/ax_boost_system.html
  # ===========================================================================
  #
  # SYNOPSIS
  # and this notice are preserved. This file is offered as-is, without any
  # warranty.
  -#serial 18
  +#serial 19
  AC_DEFUN([AX_BOOST_SYSTEM],
  [
  @@ -68,6 +68,7 @@ AC_DEFUN([AX_BOOST_SYSTEM],
  ax_cv_boost_system,
  [AC_LANG_PUSH([C++])
  CXXFLAGS_SAVE=$CXXFLAGS
  + CXXFLAGS=
  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[@%:@include <boost/system/error_code.hpp>]],
  [[boost::system::error_category *a = 0;]])],
  @@ -83,7 +84,6 @@ AC_DEFUN([AX_BOOST_SYSTEM],
  LDFLAGS_SAVE=$LDFLAGS
  if test "x$ax_boost_user_system_lib" = "x"; then
  - ax_lib=
  for libextension in `ls -r $BOOSTLIBDIR/libboost_system* 2>/dev/null | sed 's,.*/lib,,' | sed 's,\..*,,'` ; do
  ax_lib=${libextension}
  AC_CHECK_LIB($ax_lib, exit,
  @@ -108,7 +108,7 @@ AC_DEFUN([AX_BOOST_SYSTEM],
  fi
  if test "x$ax_lib" = "x"; then
  - AC_MSG_ERROR(Could not find a version of the boost_system library!)
  + AC_MSG_ERROR(Could not find a version of the library!)
  fi
  if test "x$link_system" = "xno"; then
  AC_MSG_ERROR(Could not link against $ax_lib !)
  