Warn, not fail on whitespace-only precious variable differences.
* lib/autoconf/general.m4 (_AC_ARG_VAR_VALIDATE): Output
precious variable differences less ambiguous with `ugly-quotes'.
If their settings differ only in whitespace, do not fail, but
reuse the old value.
* tests/torture.at (AT_CHECK_AC_ARG_VAR): Extend macro to allow
an optional status and expected-warning argument. Fix m4
quotation for initial value.
(AC_ARG_VAR): Also test for whitespace-only differences, and
that the old value is retained in this case.
* doc/autoconf.texi (Setting Output Variables): Document this.
* NEWS: Update.
Report and initial patch by Paolo Bonzini.