warn-on-use: work with old autoconf

warn-on-use: work with old autoconf

Before autoconf documented the AS_VAR_* interfaces, it used to
require the user to pre-expand the pushed macro name.  Newer
autoconf can see through macros, but this is one case where
breaking the rule of thumb and underquoting does not hurt.

* m4/warn-on-use.m4 (gl_WARN_ON_USE_PREPARE): Accomodate older
AS_VAR semantics of autoconf 2.60.
Reported by Bruno Haible.

Signed-off-by: Eric Blake <eblake@redhat.com>