autoconf 2.62 failure due to AS_VAR_IF

[Gerald Pfeifer]

Using autoconf 2.62 on my primary test system I am getting:

  configure:26081: error: possibly undefined macro: AS_VAR_IF

Indeed use of AS_VAR_IF appears relatively new, and it seems this may be 
similar to AS_VAR_APPEND where we have the following in configure.ac:

  dnl autoconf versions before 2.63b don't have AS_VAR_APPEND
  m4_ifdef([AS_VAR_APPEND],,[as_fn_append () { eval $[1]=\$$[1]\$[2]; }
  AC_DEFUN([AS_VAR_APPEND],[as_fn_append $1 $2])])dnl

I assume we need something similar here, too?  I am really far from
an autoconf hacker -- is this something one of you could look into?

Gerald