configure: require libzstd >= 1.4.0

configure: require libzstd >= 1.4.0

gas uses ZSTD_compressStream2 which is only available with libzstd >=
1.4.0, leading to build errors when an older version is installed.

This patch updates the check libzstd presence to check its version is
>= 1.4.0. However, since gas seems to be the only component requiring
such a recent version this may imply that we disable ZSTD support for
all components although some would still benefit from an older
version.

I ran 'autoreconf -f' in all directories containing a configure.ac
file, using vanilla autoconf-2.69 and automake-1.15.1. I noticed
several errors from autoheader in readline, as well as warnings in
intl, but they are unrelated to this patch.

This should fix some of the buildbots.

OK for trunk?

Thanks,

Christophe

config/ChangeLog:

	* zstd.m4: Add minimum version requirement of 1.4.0.