gl_CACHE_VAL_SILENT won't work correctly with upcoming autoconf 2.70

[Zack Weinberg]

I'm doing groundwork for the long-overdue release of autoconf 2.70,
and part of that is checking to make sure that various changes in
development trunk since 2.69 haven't broken third-party macros.  I
noticed that gnulib-common.m4 contains a macro, gl_CACHE_VAL_SILENT,
that attempts to make AC_CACHE_VAL not print " (cached)" by changing
the value of the shell variable $as_echo_n.  This has had no effect
with development autoconf's AC_CACHE_VAL since 2013 (!)  and since,
um, today, it will also trigger -Wobsolete warnings.

Abstractly, the best thing to do about this would be to remove the
macro and change all of its uses to be AC_CACHE_CHECK, with proper
messaging, instead.  This would only not be possible if there was ever
a gl_CACHE_VAL_SILENT invocation *nested inside* an AC_CACHE_CHECK
invocation -- which shouldn't be done anyway, since the entire thing
would be skipped if the outer test is cached.  However, reviewing the
various uses, they are complicated enough that I don't think I am the
best person to write that patch.  I may still attempt it ...
eventually ... but I have a whole bunch of other work to do to make
the release possible, and this particular problem is mostly cosmetic.

zw