configure: Replace which(1) with "has"

Using "has" is more slick because which(1) is not always there.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Amos Kong <akong@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

configure

@@ -2727,7 +2727,7 @@ fi
 if test "$modules" = yes; then
     shacmd_probe="sha1sum sha1 shasum"
     for c in $shacmd_probe; do
-        if which $c >/dev/null 2>&1; then
+        if has $c; then
             shacmd="$c"
             break
         fi