libevent: fix openssl detection when statically linking

Fixes (indirectly):
http://autobuild.buildroot.net/results/dbf/dbf2d348de3831a86728f9d72cb56b9f2ddbf554/
http://autobuild.buildroot.net/results/6a8/6a89183e558e4ff27841c3e544687ad4a04fb036/
http://autobuild.buildroot.net/results/c80/c80eedb4d7335f1aef736a1312900c2b371eabc9/
http://autobuild.buildroot.net/results/91b/91b4ab54ed9d422bfaef765d09cc92192012ce7e/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libevent/libevent.mk

@@ -26,6 +26,11 @@ endif
 ifeq ($(BR2_PACKAGE_OPENSSL),y)
 LIBEVENT_DEPENDENCIES += openssl
 LIBEVENT_CONF_OPTS += --enable-openssl
+# openssl needs zlib but configure forgets to link against it causing
+# openssl detection to fail
+ifeq ($(BR2_STATIC_LIBS),y)
+LIBEVENT_CONF_ENV += OPENSSL_LIBADD='-lz'
+endif
 else
 LIBEVENT_CONF_OPTS += --disable-openssl
 endif