configure: Improve zstd test

There were one error on the test (missing an s for --exists).
But we really need a recent zstd (1.4.0).
Thanks to Michal Privoznik to provide the right vension.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reported-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200310111431.173151-1-quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

configure

@@ -2475,7 +2475,8 @@ fi
 # zstd check
 
 if test "$zstd" != "no" ; then
-    if $pkg_config --exist libzstd ; then
+    libzstd_minver="1.4.0"
+    if $pkg_config --atleast-version=$libzstd_minver libzstd ; then
         zstd_cflags="$($pkg_config --cflags libzstd)"
         zstd_libs="$($pkg_config --libs libzstd)"
         LIBS="$zstd_libs $LIBS"