[PATCH] configure: Fix configure error.

[Zhang Chen]

From: Zhang Chen <address@hidden>

When run the ./configure will always get this error:
Unknown option --exist

It caused by this patch:
commit 3a67848134d0c07da49033f9ed08bf0ddeec0c6d
Author: Juan Quintela <address@hidden>
Date:   Tue Dec 17 21:15:24 2019 +0100

    configure: Enable test and libs for zstd

    Add it to several build systems to make testing good.

Signed-off-by: Zhang Chen <address@hidden>
---
 configure | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/configure b/configure
index 3c7470096f..305591f7e0 100755
--- a/configure
+++ b/configure
@@ -2475,7 +2475,7 @@ fi
 # zstd check
 
 if test "$zstd" != "no" ; then
-    if $pkg_config --exist libzstd ; then
+    if $pkg_config --exists libzstd ; then
         zstd_cflags="$($pkg_config --cflags libzstd)"
         zstd_libs="$($pkg_config --libs libzstd)"
         LIBS="$zstd_libs $LIBS"
-- 
2.17.1