syslog-ng: Specify location of libesmtp

If --with-libesmtp is omitted a host tool is invoked to detect the
location of the library and headers, which is obviously the wrong thing
for a cross-compile environment. Explicitly add --with-libesmtp to
resolve this.

Fixes:
http://git.buildroot.net/buildroot/commit/?id=539acb7500588d0a277357cf831c9f25f770578d
http://git.buildroot.net/buildroot/commit/?id=01f41585a78406f6e830d5eaf4413a983812a299

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/syslog-ng/syslog-ng.mk

@@ -37,6 +37,7 @@ endif
 ifeq ($(BR2_PACKAGE_LIBESMTP),y)
 SYSLOG_NG_DEPENDENCIES += libesmtp
 SYSLOG_NG_CONF_OPTS += --enable-smtp
+SYSLOG_NG_CONF_OPTS += --with-libesmtp="$(STAGING_DIR)/usr"
 else
 SYSLOG_NG_CONF_OPTS += --disable-smtp
 endif