libssh2: Fix build with autoconf 2.70+

Fixes
 configure.ac:130: error: m4_undefine: undefined macro: backend

Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-oe/recipes-support/libssh2/files/0001-configure-Conditionally-undefine-backend-m4-macro.patch

@@ -0,0 +1,30 @@
+From efe7101786193eaddb749c0583af6b54aec6f289 Mon Sep 17 00:00:00 2001
+From: Khem Raj <raj.khem@gmail.com>
+Date: Tue, 2 Feb 2021 18:45:16 -0800
+Subject: [PATCH] configure: Conditionally undefine backend m4 macro
+
+Unlike the M4 builtin, this macro fails if macro is not defined
+therefore recover the behavior of the builtin.
+
+Upstream-Status: Pending
+Signed-off-by: Khem Raj <raj.khem@gmail.com>
+---
+ configure.ac | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/configure.ac b/configure.ac
+index fe5054a..758f8c2 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -127,7 +127,7 @@ fi
+ m4_set_foreach([crypto_backends], [backend],
+   [AM_CONDITIONAL(m4_toupper(backend), test "$found_crypto" = "backend")]
+ )
+-m4_undefine([backend])
++m4_ifdef([backend], [m4_undefine([backend])])
+ 
+ 
+ # libz
+-- 
+2.30.0
+

meta-oe/recipes-support/libssh2/libssh2_1.9.0.bb

@@ -9,6 +9,7 @@ LIC_FILES_CHKSUM = "file://COPYING;md5=c5cf34fc0acb44b082ef50ef5e4354ca"
 
 SRC_URI = "http://www.libssh2.org/download/${BP}.tar.gz \
            file://CVE-2019-17498.patch \
+           file://0001-configure-Conditionally-undefine-backend-m4-macro.patch \
            file://run-ptest \
 "