package/syslog-ng: fix libcurl build without C++

Fix the following libcurl build failure without C++ raised since bump to
version 4.7.1 in commit 105f0398c76f3288b2668e0b3ec722b2ebe55a0b and
https://github.com/syslog-ng/syslog-ng/commit/86feed7bff068a48be477a5b702ca24abc441d70:

configure: error: C++ support is mandatory when the cloud-auth module is enabled.

Fixes: 105f0398c76f3288b2668e0b3ec722b2ebe55a0b
 - http://autobuild.buildroot.org/results/cf36fb119224ba50a768dbe363d4c381f76eb476

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

package/syslog-ng/syslog-ng.mk

@@ -61,8 +61,13 @@ endif
 
 ifeq ($(BR2_PACKAGE_LIBCURL),y)
 SYSLOG_NG_DEPENDENCIES += libcurl
-SYSLOG_NG_CONF_OPTS += --enable-http --enable-cloud-auth
+SYSLOG_NG_CONF_OPTS += --enable-http
 SYSLOG_NG_CONF_OPTS += --with-libcurl="$(STAGING_DIR)/usr"
+ifeq ($(BR2_INSTALL_LIBSTDCPP),y)
+SYSLOG_NG_CONF_OPTS += --enable-cloud-auth
+else
+SYSLOG_NG_CONF_OPTS += --disable-cloud-auth
+endif
 else
 SYSLOG_NG_CONF_OPTS += --disable-http --disable-cloud-auth
 endif