Correcting warnings in Autoconf 2.70

[Mohammad Akhlaghi]

Dear Maintainers of Autoconf archives,

Recently (after updating my Autoconf to version 2.70), I noticed a bad 
warning from autoconf while bootstrapping Gnuastro, about using 
'as_echo' in the 'ax_pthread' module.

I have corrected it manually in Gnuastro's 'bootstrap.conf' file:

http://git.savannah.gnu.org/cgit/gnuastro.git/commit/?id=0bd07e68d57

But it would be good if this is corrected upstream, and possibly in 
other modules also, especially as more users will be migrating to 
autoconf 2.70 shortly.

Cheers,
Mohammad